1. A quantitative change, especially a small or incremental
one (this use is general in physics and engineering). "I just
doubled the speed of my program!" "What was the delta on
program size?" "About 30 percent." (He doubled the speed of
his program, but increased its size by only 30 percent.)

2. [Unix] A diff, especially a diff stored under the set
of version-control tools called SCCS (Source Code Control
System) or RCS (Revision Control System). See change
management.

3. A small quantity, but not as small as epsilon. The
jargon usage of delta and epsilon stems from the
traditional use of these letters in mathematics for very small
numerical quantities, particularly in "epsilon-delta" proofs
in limit theory (as in the differential calculus). The term
delta is often used, once epsilon has been mentioned, to
mean a quantity that is slightly bigger than epsilon but
still very small. "The cost isn't epsilon, but it's delta"
means that the cost isn't totally negligible, but it is
nevertheless very small. Common constructions include "within
delta of ---", "within epsilon of ---": that is, "close to"
and "even closer to".