Plural of buildup; accumulations or increases of something, or the process of gradually increasing or strengthening something.
From 'build up' (phrasal verb) plus '-up' (forming nouns from verbs), pluralized with '-s'. The term became common in 20th-century technical and scientific vocabulary.
Phrasal verbs like 'build up' create nouns that are super useful—'buildup' works perfectly for everything from tension in a story to plaque on teeth. English speakers constantly invent new nouns this way without formal permission.
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