δ

Wiktionary

1. n. mathematics A small increment or change in a variable.
2. n. 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.
3. n. mathematics A small number, usually accompanied by ε.