A Greek letter, (δ: lower case, and Δ: upper case). Δ is often used
to represent a difference, usually a small difference. In calculus you talk about
epsilon/delta ε/δ arguments for calculating the limit of a series. You
might also talk about delta files — the files that contain the differences
between two sets of files. These are typically much smaller than either set of files,
and can be transmitted more efficiently. JUnit has a method assertEquals( floatexpected, floatactual, floatdelta). Here the word delta means the
tolerance, how close the numbers have to be to be considered equal.