1. [techspeak] 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).

3. n. 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’.