na-adjective
unique; one-to-one; unambiguous
Used in mathematics, logic, and formal contexts to describe a mapping, solution, or interpretation that is uniquely determined. Not a general-purpose word for 'unique' in the sense of 'special' or 'unusual'.
この方程式には一意的な解が存在する。
This equation has a unique solution.
写像が一意的であることを証明せよ。
Prove that the mapping is one-to-one.
ユニーク is the common loanword for 'unique' in the sense of 'distinctive' or 'special', whereas 一意的 is a technical term for mathematical or logical uniqueness.
Derived from 一意 (いちい, 'one meaning' or 'single-mindedness') + 的 (てき, adjectival suffix). The technical sense of 'unique' or 'unambiguous' likely developed from the idea of a single, well-defined interpretation.