同类 literally means 'same kind'—composed of 同 (tóng, 'same') and 类 (lèi, 'kind, category, type'). It refers to things or beings that share essential characteristics, belong to the same classification, or are comparable in nature, function, or status. As a noun, it often implies equivalence or peer-level relationship rather than mere similarity.

The term is commonly used in scientific, academic, or formal contexts—for example, when discussing animal species ('birds and mammals are not同类'), social groups ('people of the same profession are同类'), or abstract concepts ('two theories may be considered同类'). While occasionally used colloquially, it carries a neutral-to-formal register and seldom appears in casual speech without contextual framing like 是同类 or 属于同类.