periostitis
Periostitis is the inflammation of the periosteum. It is a painful condition that may involve mild swelling and tenderness in the affected area. It often is associated with medial tibial stress syndrome (sometimes also referred to as “shin splints”), which commonly affects runners.
The word apogee is a noun. It is an astronomy term.
The word biodiversity is a noun. It is a term in the science biology.
cholecystitis
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hi
The word hydrofoil is a noun. It is a nautical term.
Since the word 'term' is a common noun, you only capitalize it if at the start of a sentence or part of a name/title.
The word uncle is a singular noun. The plural term is uncles.
The word repayment is a singular noun. The plural term would be repayments.
The word antiair (one word) is an adjective. It means antiaircraft which is a military term.
The term for knowing a word's part of speech in order to appropriately use it in a sentence is "word-usage."