GateA gate is a point of entry to a space enclosed by walls, such as a castle or fortified town, or the actual doors that block entry through the gatehouse.
In ancient and medieval times, gatehouses of cities and castles were heavily defended and fortified to prevent breaching of the gates. Often the gate would consist of several pairs of doors and iron grates along a tunnel through the gatehouse. The top of the tunnel commonly had murder holes to allow defenders to attack invaders trying to breach the inner doors. Drawbridges were common in conjuction with gates to facilitate passing the moat; moats were often used to increase the effective height of the walls.
In a transistor, particularly a field effect transistor or FET, the Gate is the controlling terminal to which a voltage is applied to control the current passing between the Source and Drain.
In the context of electronic digital logic design, a gate is a logic function building block, such as and, or, not, nand, nor, xor (exclusive-or). For measuring the complexity of logic circuits such as ASICs, the amount of logic they contain is compared to a number of 2-input nand or nor gates.
Depending on technology, a 2-input nand or nor gate requires 4 or 6 transistors.
An edge-triggered D-type flip-flop is usually considered equivalent to 6 gates, more if it has clock enable and reset functionality.
See also: logic gate.