Something that causes or produces alteration or change.
From Latin alterans, present participle of alterare (to change), plus -ant (suffix denoting an agent or doer). This medical/pharmaceutical term entered English in the 16th century for substances believed to alter bodily humors.
Medieval physicians prescribed 'alterants'—herbal medicines supposedly able to fundamentally transform your body's balance of hot/cold and wet/dry. Today we'd call them tonics or adaptogens!
Complete word intelligence in one call. Free tier — 50 lookups/day.