Word-internal deletion. An example of optional syncope in English is the pronunciation of asked as ɑːst. In many languages the addition of a derivational or inflectional suffix often causes obligatory syncope of a sound in the root. An example from Irish: obair (“work”) ˈobir, plural oibreacha ˈobʲrʲəxə.
The term is sometimes used rather loosely to mean any sort of deletion.