A tithe is a portion of one's earnings, usually one-tenth, that is given to those who perform the work of the Lord since it belongs to the Lord (Lev. 27:30-33). Those who received tithes in the OT consisted of priests (Num. 18:21-32). Further OT references are Gen. 14:20, 28:22, 2 Chron. 31:5f, Mal. 3:7-12).
In the NT there is no command to tithe a tenth (since we are not under law but grace). But the tithe is mentioned in Luke 18:9-14, 1 Cor. 16:1, and 2 Cor. 8).