[see delta]

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."