A substance or agent that causes or produces something; in some historical or technical contexts, something that goes or moves (archaic).
From Old French aller, from Latin ambulare (to walk). The modern sense is mostly obsolete, though it survives in compound words and technical terminology.
The word 'aller' is basically hiding inside hundreds of English words you use every day—'ambulance' comes from the same Latin root, and 'aller' originally meant 'one who walks or goes'!
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