In modern usage, a relict is a surviving remnant of some natural phenomenon. This is perhaps most often used in biology: a relict species is a species that was once common, but now survives in only a small area; a relict ecosystem is one that that is shrinking and being replaced by other ecosystems.
In much the same vein, in geology a relict is an isolated rock that survived a destructive geological process, e.g., a pocket of mineral from a parent rock that did not undergo metamorphosis when the surrounding rock did. It is used more broadly in the field of geomorphology, in which a relict landform is any that was produced by processes that are no longer active; this extends, sometimes, even to refering to extinct (and well-eroded) volcanoes as 'relict volcanoes'.
Webster1913 (below) refers to a legal definition than has changed a bit over time; although the term is no longer commonly used, it is applied to the survivor of a pair of married people, whether widow or widower; it is usually used in the context of inheritance law. It comes to us from Scots law, which usually uses the technical Latin terms jus relictae for inheritance by a widow and jus relicti for inheritance by a widower.