A packaging format for effects and some guitar synthesizer units, intended to be placed on the floor and operated by the performer's feet. A stompbox usually consists of a heavy gauge metal or plastic box, with one or more foot switches or pedals on top, as well as perhaps some other knobs and switches (for which the perfomer must sit or kneel on the floor to adjust). Input and output jacks are usually along the front edge of the box, to minimize the possibility of them being stepped on by the performer.
Stompboxes are most commonly used to package effects for guitar players. Although guitar effects definitely have their uses with synthesizers, there will often be signal level matching problems; the output level of a passive guitar pickup is much lower than the typical line level output of a synth, and stompboxes usually have high-impedence inputs, which prevent "loading" the guitar pickup and causing a loss of fidelity, but such inputs can easily be overdriven by a synthesizer output. Conversely, the stompbox output will often be too low to properly drive an input of a mixer. However, since 2010 or so, an increasing number of stompbox manufacturers have begun providing a line-level input and output capability on their boxes, to accommodate synthesizers and other keyboards.
Many stompboxes are powered by 9-volt batteries. As such, even ones with line-level inputs and outputs usually cannot handle the input and output levels of a typical modular synthesizer, which have voltage swings of 10V or more . Some modular synth manufacturers offer an "effects interface" module that the stompbox connects to; the interface module handles the signal level conversions, and usually provides a few other features like gated bypass or a dry/wet mixer.