The Oxford English Dictionary has a variety of contexts and definitions.

http://www.oed.com/view/Entry/202100?result=1&rskey=X9bvNX&(mar 2013)

A noun,

time considered as a period

A finite extent or stretch of continued existence, as the interval separating two successive events or actions, or the period during which an action, condition, or state continues; a finite portion of time (in its infinite sense: see sense A. 34a); a period. Freq. with preceding modifying adjective, as a long time, a short time, etc

A specific period of time

With of or (more commonly) genitive. The space of a specified period of time. Esp. after in (also within) indicating a limit of time

So time is a period, separating two successive events. A period, presumably, in which no other relevant events occur (else why would you register it). It uses interval too; there is a separate section of this book that re-analysis interval.

(We’ll ignore for now the fact that “period” is defined as an interval or portion of time, as mentioned. circularity is a common problem in time definition). It goes on:-

a. A particular period indicated or characterized in some way, either explicitly (usu. with of) or by anaphoric reference (as at the time, etc.).

This definition explains time as a means of referencing.

With the. Used in various expressions to indicate the extent to which an action, state, etc., takes place, occurs, or endures, or has always been the case, as all the time, much of the time, etc.

This is really a secondary definition, which just explains time as used in certain phrases or context by defining those phrases; but it is a use of the mass noun loosely for referencing a non-specific set of events.

a. Usu. with possessive. The period during which a person or thing lives, occupies a particular position, is active in a particular sphere, exercises influence or dominance, etc.; (sometimes) spec. the lifetime of a person or animal. Also: one’s lifetime up to the present (esp. in in one’s time).

The orbital period of a celestial object; = PERIOD n. 7b. Now chiefly in periodic time n. (a) at PERIODICadj.1 and n. Special uses.

This is again a secondary usage to reference a specific period. The next few definitions on the OED are secondary uses of the word.

However the tenth definition is more interesting:-

The fundamental quantity of which periods or intervals of existence are conceived as consisting, and which is used to quantify their duration.

It calls time a fundamental quantity (whatever that means); periods or intervals consist of it. And it is used to quantify their duration. Quantify means measure; so it measures duration then? “Periods of existence” seems elaborate. I’m not sure what this tells us that No.1 definition didn’t. But it suggests time is a measurement (but a measurement of a period, which is an interval of time – so another circular reference). Well, we can take away ‘measurement’ from this, I think.

The amount of time taken up by something; duration. Freq. with of.

A point of time; a moment in time; a space of time considered without reference to its duration; an occasion, an instance. The ‘point’ may be an instant (as the time when a star crosses the meridian), or it may have some duration (as the time for sowing), but the focus of consideration is not on its extent or duration but rather on the question of when it occurs (i.e. where it is situated within a greater space or period of time), what happens or is done at that moment, or how it is characterized.

This brings up duration again, and time as a referencing system.

A point in the course of time or of a period or cycle. Cf. TIDE n. 3a.

This seems to be a circular definition, though perhaps recognising that time is a referencing system.

A point or fixed part of the year, a season. In later use chiefly in time of year, or with preceding modifying word.

(a) A point or period in the course of a day, or of the diurnal cycle, as determined or described according to some system of measurement, or as shown by a clock;

Again, referencing and calibration (e.g. seconds, minutes hours etc.)

Duration conceived as beginning and ending with the present life or the material universe, or as the sphere within which human affairs are contained; finite duration as distinct from eternity; the duration of the world or universe.

This says nothing really, though is grand in its scope – “the duration…of the universe.” It still doesn’t tell us what time is as a concrete thing.

34.a. Indefinite continuous duration regarded as that in which existence, and the sequence of events, takes place; the abstract entity which passes, goes by, or is consumed as events succeed one another, esp. in regard to the bringing about of anticipated developments, change, etc.

This final definition is interesting. It calls time a duration in which the sequence of events takes place. And it says it’s an abstract entity which passes as events pass…bringing change.

The Oxford Dictionary (different to the Oxford English Dictionary) has this:-

http://oxforddictionaries.com/definition/english/time?q=time (mar 2013)

[Mass Noun]: the indefinite continued progress of existence and events in the past, present, and future regarded as a whole

What is helpful here is that time is described as a mass noun. A mass noun (or uncountable noun) is a noun that references a genre or class singularly. So Art is a mass noun, or Furniture, or Wine (for example, “I like Art” doesn’t refer to any specific piece of art). I think this is the definition the OED would have like to have got to in their 34th attempt above.

A characteristic of mass nouns is that they are indefinite, or unspecific, composites of underlying items.

Furniture (as in the mass noun usage “I like furniture”) refers to an indefinite set of chairs, tables, drawers etc. Water as a mass noun (as in “I drink water”), refers to an indefinite, unspecified, set of water. This might seem circular, but we know that we are referring to a (unspecific) collection of H2O molecules, even if that seems extreme.

Time as a mass noun refers to what? What are the individual items that underlie the class or set being referred to ? Little individual bits of time? That certainly is circular. We need be able to define time outside of itself.

The definition above calls time a mass noun, and describes it as the “…progress of existence and events” –which is an elaborate way of saying “change”. In the attempt at the definition no. 34 the OED refers also to sequence of events and change.

So is time a mass noun referring to events and/or change? We have encountered references to events, periods and duration in the definitions so far. Time, as a mass noun, could be referring to events, durations and periods? “Let’s go back in time” suggests going back to a time before subsequent events, durations and periods had occurred. That seems to work. So time is a mass noun referring to events and duration (i.e. incidences of change).

The Merriam-Webster dictionary has a different perspective on time:-

http://www.merriam-webster.com/dictionary/time (mar 2013)

1 a : the measured or measurable period during which an action, process, or condition exists or continues : duration

b : a nonspatial continuum that is measured in terms of events which succeed one another from past through present to future

Well, this brings in measurement again. It calls time a “measured period”. And also defines it as something that is “measured in terms of events”. Is time measured, or is it the measurer?

Dictionary.com has yet another view:-

http://dictionary.reference.com/browse/time?s=t&path=/

a system or method of measuring or reckoning the passage of time

So time is a system of measuring? Sure, it goes on to say,” measuring the passage of time”, so its circular. But that time is a measurement is key. We just need to determine what it measures, other than itself.

The Free Dictionary by Farlex has some core definitions:

http://www.thefreedictionary.com/time(mar 2013)

A nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.

An interval separating two points on this continuum; a duration: a long time since the last war; passed the time reading.

A number, as of years, days, or minutes, representing such an interval: ran the course in a time just under four minutes.

A similar number representing a specific point on this continuum, reckoned in hours and minutes: checked her watch and recorded the time, 6:17 a.m.

A system by which such intervals are measured or such numbers are reckoned: solar time

The first definition is (a sub-set of) the mass noun that we’ve already mentioned.

The second refers to interval and duration. The third recognises time as a calibration of interval/duration. The fourth is time as a referencing system. And the fifth consolidates calibration and referencing into a system – in other words a measurement.