A low-level digital logic component. Gates perform Boolean functions (e.g. AND, NOT), store bits of data (e.g. a flip-flop), and connect and disconnect various parts of the overall circuit to control the flow of data (tri-state buffer).
In a CPU, the term applies particularly to the buffers that route data between the various functional units. Each gate allows data to flow from one unit to another or enables data from one output onto a certain bus.