δ

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mathematics A small increment or change in a variable.
• noun mathematics Denotes a small — rather infinitesimal — variation of the value returned by a functional and/or of the functions on which that functional depends. This symbol can also — along a similar vein — be used to help denote virtual displacement and virtual work.
• noun mathematics A small number, usually accompanied by ε.