What is time? Is it a measurement of something? If so, what is it a measurement of?
Time is the measurement of…change.
Seriously, if you look in a physics book, time is how we tell things change.
Ultimately, we really have no friggen idea. Change is what it measures, not what it is. We have lots of ways of decribing what it does, and how it does it…but that falls short to me somehow.
Interstingly, all objects move through space/time at the same speed, the speed of light©. An immobile object has speed c through time, and 0 speed through space. An object moving at speed c through space moves at speed 0 through time.
OK, OK, since no one else will, I’ll quote it.
Time is what keeps everything from happening at once.
This site has an interesting discussion on the subject. This is the opening paragraph.
Time and space are both part of the same thing and one cannot exist or be explained or understood without the other. The question “what is space?” is just as difficult to answer.
What is space? - What I need more of in my garage.
What is time? What I need more of to tidy the garage.
See? They are both related to the garage. 
Time is what keeps everything that will ever happen to a single point in space from happening all at once. Or Time is what keeps on slipping into the future…
This whole topic could be beaten to death in GD if anyone felt up to it.
Since I consider it my duty to mention Brian Greene at least once in every thread, here it is:
His new book, The Fabric of the Cosmos: Space, Time, and the Texture of Reality, contains an entire section on the conundrum that is time, and what scientists latest thoughts on the subject are.
The human brain perceives notions of spatial position (where something is along x, y and z axes) and of identity (that one object is or is not the same as another, even if it LOOKS similar). Time is the conceptual label we give to our perception that our notions of spatial position, and of identity, require one more factor if we are to achieve consistency of perception.
There is a cup on the left of the table. I move it, so it is now on the right of the table. Without the concept of time, I have to posit that one cup can be in two different spatial positions, or that two cups with two different spatial positions (the one on the left and the one on the right) are in some sense the same, and share one identity.
Invoking the concept of time resolves both these perceptual issues, and allows for consistency of perception. The human brain is wired up to prefer this way of perceiving things and to experience problems otherwise.
At a more deep and profound level, time is what slows down to a near standstill when people want to tell me about their friend who had an experience that proves there really are genuine psychic powers, and what speeds up when I’m enjoying the finest Rombauer Zinfandel with a deliciously attractive female friend.
7:12 pm, next question 
Paul Davis’s book About Time is a pop-sci book that tries to tackle this question. Defintely worth a read.
Time is a degree of freedom in a system.
Time is…
Too Slow for those who Wait
Too Swift for those who Fear
Too Long for those who Grieve
Too Short for those who Rejoice
But for those who Love
Time is not.
Time is that agency that ranks causes before effects. It appears to flow or have substance or meaning by itself because we perceive it with our brains, and the only job of brains is the manipulation of causes to achieve effects.
Grey took my answer: Time is what keeps all things from happening at once.
Of course I stole that answer from the Official Handbook of the Marvel Universe.
That’s beautiful,
SQUINK…got an author for that?
What speed through time does a hypothetical superluminal particle, the tachyon, go?
I’m of the considered opinion that time slips into the past for normal people. Thus the authors of this quote are misnamed. They are the Steve Miller Anti-Band.
The particle would also move at c. All 4-velocities have the same length (plus or minus one in geometrized units, depending on your sign convention).
A bit of a hijack, but the really fascinating thing about tachyons is that the particle velocity is superluminal, but the packet velocity is still subluminal. That is, individual tachyons travel faster than light, but no signal composed of them can travel faster than light.
slurpslurpslurp 
URL=http://www.cyberhymnal.org/bio/v/a/vandyke_hj.htm]Henry Jackson Van Dyke
The poem was read at princess Diana’s funeral.
It also showed up, in mutant form, in a 60’s rock song -can’t remember the name of the band.