SinkSink and source are respectively the termination or supply of current flow; the divergence of the current density is negative and positive, respectively. More generally this can be applied for other vector fields than current density; for example, mass is a sink of gravitational field strength.
A TTL (Transistor Transistor Logic)circuit will "sink" current when the circuit is a a logic "0" (grd) and "source" current at a logic "1" (+5vdc).
[These terms are far more generally used than just in digital electronics. Indeed, they antedate digital electronics. They are used in thermodynamics, which originated in the 19th century, and also for other physical systems such as rivers. Would some physicist complete this article?]