what is meant by a "measurement" in quantum mechanics?

Answer 1

What meant Indeed ?

It is not a easy question. This question was originally highlighted by Niels Bohr in the Copenhagen interpretation, where a system stops being a superposition of states and becomes either one or the other when an observation takes place. This makes apparent the fact that the nature of measurement, or observation, is not well defined. In other words we can measure position, but not the velocity or we can measure velocity, but not the position.

I did mention Schrödinger's cat did not I? The Schrödinger's cat paradox suggests that by the act of quantum measurement we are destroying the object of measurement and hopefully not before we obtain one particular property.
http://en.wikipedia.org/wiki/Schr%C3%B6dinger%27s_cat

So by ‘measurement’ in QM we mean “In classical physics the mathematical model talks about the things we observe. In QM the mathematical model by itself never produces observations. We must interpret the wave function in order to relate it to experimental observations.” http://www.mtnmath.com/faq/meas-qm-2.html

I hope that tickles you, but I suspect you knew the answer.

Answer 2

That's pretty broad. Are you sure there isn't more to it, such as "Velocity measurement" or "Measured mass"?