一定量 is a noun meaning 'a fixed quantity' or 'a certain amount'. It is a compound of 一定 ('fixed, given') and 量 ('quantity, amount'). You use it when referring to a specific but often unspecified amount of something — like a dosage, a portion, or a threshold — that is predetermined or required by context. It is more formal and quantitative than 一些 ('some') and appears in technical, medical, or procedural writing.