Here's what dictionary.com has to say:
1. quantity or amount: the least quantum of evidence.
2. a particular amount.
3. a share or portion.
4. a large quantity; bulk.
a. the smallest quantity of radiant energy, equal to Planck's constant times the frequency of the associated radiation.
b. the fundamental unit of a quantized physical magnitude, as angular momentum.
6. sudden and significant: a quantum increase in productivity.
I was reminded of this word by this wonderful quote:
"There is an art in writing for the Theatre, technically called touch and go, which is indispensible when we consider the small quantum of patience, which so motley an assemblage as a London audience can be expected to afford."
Rejected Addresses, by H Smith and J Smith, 1812