Short for Linux Audio Developer's Simple Plugin API.
LADSPA is a plugin standard for audio programs in Linux, sort of similiar to VST plugins. Basically, you can write your fancy effect or whatever once, and the plugin can be then used in all LADSPA-compliant programs.
Couple of examples that use LADSPA include Ecasound, a really amazing command-line sound mangler; Sweep, a nice sample editor, and Ardour, a digital audio workstation.
(An interesting note: I noticed that Sweep, without any plugins, couldn't do a damn thing, not even a fade in/out. So, thoughtfully, I installed "cmt" and "swh-plugins" packages that have tons of LADSPA frostiness. Okay, so now Sweep can do zillions and zillions of things, but guess what? No mention whatsoever of fadein/out! =( )
Home page: http://www.ladspa.org/