Boyd’s Bible Dictionary:

An annual of the parsley family, producing aromatic seeds (Isa. 28:25; Matt. 23:23).

Concise Bible Dictionary:

A plant yielding a small aromatic seed, used as a condiment and for medicines. It is beaten out by a rod, and is one of the bountiful gifts of God (Isa. 28:25,27). The Pharisees paid tithes of it, whereas they omitted the weightier matters of the law, judgment, mercy and faith (Matt. 23:23).
Black Cumin