辅助机车又叫辅机或补机，是当铁路列车行驶上坡道、天候因素或故障，致使铁路机车超过牵引定数无法独力牵引列车时，用以辅助该列车行驶之铁路机车。

依据台湾铁路管理局行车实施细则第一章总则，第二条本细则用语，释意如左：
二十三. 辅助机车：指因列车行驶上坡道等，本务机车超过牵引定数时用以辅助该列车行驶之机车。

A bank engine (United Kingdom/Australia) (colloquially a banker) or helper engine or pusher engine (North America) is a railway locomotive that temporarily assists a train that requires additional power or traction to climb a gradient (or bank). Helpers/bankers are most commonly found in mountain divisions (called "helper districts" in the U.S.), where the ruling grade may demand the use of substantially greater motive power than that required for other grades within the division.