A circuit board packaging technique in which the leads (pins) on discrete components and chips are inserted through holes in the printed circuit board and soldered from beneath. Until the late 1980s and prior to surface mount technology, all devices on circuit boards were thru-hole. Although chips are mostly surface mounted these days, some chips are still thru-hole, and most discrete devices such as resistors and capacitors are thru-hole.
Thru-hole devices have a strong bond with the board but require an extra drilling step. They also eliminate the board real estate underneath from being used for other layers. See surface mount
Through the Board
In this circuit board from an LED table clock made in 2005, not only the discrete components, but the chip, are thru-hole devices.
Thru-Hole Vs. Surface Mount
The thru-hole chip is not only thicker than the surface mount chip, but it eliminates the real estate underneath because the pins stick through. The board designer must route any traces from other layers around that part of the board.