A delta update is a software update that requires the user to download only those parts of the software's code that are new, or have been changed from their previous state, in contrast to having to download the entire program. The use of delta updates can save significant amounts of time and computing bandwidth. The name "delta" derives from the mathematical science use of the Greek letter delta, Δ or δ to denote change.[1]
delta...means 'change' or 'the change in' in mathematics