American Dictionary of the English Language

SYNON'YMOUS, adjective Expressing the same thing; conveying the same idea. We rarely find two words precisely synonymous Wave and billow are sometimes synonymous but not always. When we speak of the large rolling swell of the sea, we may call it a wave or a billow; but when we speak of the small swell of a pond, we may call it a wave, but we may not call it a billow.