The present participle of 'deaerate'; the ongoing process of removing air from a substance.
From 'deaerate' + '-ing' suffix marking the continuous or gerund form, a productive pattern in English for creating verbal nouns and present participles.
You'll find 'deaerating' in lab manuals describing meticulous steps—the '-ing' form shows this is an active, careful process that must be done exactly right.
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