1. n. A small quantity of anything.
"The cost is epsilon."
2. adj. Very small, negligible; less than marginal.
"We can get this feature for epsilon cost."
3. 'within epsilon of': close enough to be indistinguishable for all practical purposes. This is even closer than being 'within delta of'.
"That's not what I asked for, but it's within epsilon of what I wanted."
Alternatively, it may mean not close enough, but very little is required to get it there:
"My program is within epsilon of working."