(usually pluralized, chiefly philosophy) Something which has exactly the same properties as something else.

An identical twin.

(poetry) A rhyme on the same syllable, such as "leave" and "believe".

(not comparable) Bearing full likeness by having precisely the same set of characteristics; indistinguishable.

(not comparable) Not different or other; not another or others; not different as regards self; selfsame; numerically identical.

(not comparable, biology) Of twins, sharing the same genetic code.

(not comparable, mathematics) Exactly equivalent.

(comparable, rare) Approximating or approaching exact equivalence.