A software "patch" is a "quick fix" -- generally a change of limited scope to fix bugs, vs a change to provide new function.
Precisely how the "patch" is presented (eg, in source code or object code) is not implied by the term.
(The term most likely was borrowed from hardware engineers, who would "patch" hardware to correct bugs by, eg, cutting wires in the backplane and soldering in new ones. Originally programmers would perform the analogous surgery by using the switches and lights on the control panel to change memory locations containing hardware instructions. But the term "stuck" to the concept of a "quick fix" even though the mechanism for applying the patch got further and further removed from the actual hardware.)