Though it sounds fancy, it’s really quite simple: aspic is savory gelatin.  Aspic used to be used more often in European cuisine, but is now rather rare.  It’s usually made from gelled stock, although any liquid can be used.  Some varieties of stock will gel on their own if cooled, while other liquids will need extra gelatin dissolved in them first.  The texture of aspic can be smooth, silky, and delicate, or firm (aka sliceable aspic) like gelatin desserts.

In the past, aspic was sometimes used in elaborate decorative platters, but now is most often used as a binder for cooked meats like headcheese, or to add shine to the surface of traditionally made pates & mousses.