To make less, or to bring many down to one. Reduction always discards, and what it discards should be named.
Latin reducere (to lead back, to bring back): re- (back) + ducere (to lead); to reduce was first to lead something back to a prior state, and the sense of bringing many down to one, or making less, followed from that leading-back.
To make less, or to bring many down to one. The root's leading-back is a fair warning: reduction always discards something along the way, and what it discards should be said plainly.