an act or instance of filling.

something that is put in to fill something else:

Dentistry. a substance such as cement, amalgam, gold, or the like, used to fill a cavity caused by decay in a tooth.

a food mixture that goes into something, as if to fill it:

Also called pick, weft, woof. Textiles. yarn carried by the shuttle and interlacing at right angles with the warp in woven cloth.