The term "outboard" is usually used in the context of "outboard equipment," "outboard gear," or "outboard processor" as a way to distinguish a piece of audio equipment that was not built into the mixing console of a recording studio. The mixing console was also commonly referred to as the "mixing board" or "board."
The earliest forms of outboard gear were limiter-compressors used to automatically control the volume of audio for a variety of reasons. Occasionally special equalizers were employed to give more important musical instruments or vocals a pleasing "tone."
In the 1970's, the first outboard effects processors started to appear such as Phase-shifters and "delay" units designed to add ambient effects in addition to the more common "reverb." As digital technology evolved, multi-effects processors capable of generating a number of effects such as flanging, chorus, delay, and reverberation became available. It was common for outboard gear to be made in the 19" rack form as a way to simplify integrating them into the studio environment.
Today, most outboard processing has been replaced by software "plug-ins" that duplicate the same effects as well as perform new ones. However; the outboard effects unit still has the advantage of operating (virtually) in "real-time;" so there are still application such as live sound where outboard processors can still be found. Most employ high-powered DSP processors that are capable of performing large numbers of complex calculations faster than personal computers which are optimized for other applications.