Minimal is an adjective, and as such must be used to sharpen the style description (minimal house, for instance). Minimalism has an aesthetic which can be summarized as "less is more", like cold-wave bands looking for an empty sound in the 80's.
When used by itself, it generally denotes music that uses only one instrument, without complex layers. Usually no vocals layer.
Note that there is a separate 'minimal techno' tag. One hopes that users will gradually adjust techno releases with the 'minimal' tag to 'minimal techno' .