1.
lucid, clear, clear-headed
Describes a mind, thinking, or logic that is sharp, clear, and well-organized. Often used to praise someone's intellectual clarity.
頭脳明晰な人だ。
A person with a sharp, clear mind.
高齢でも意識は明晰だった。
Despite being elderly, the person's mind was lucid.
彼女の論理は常に明晰で、聞いている人を納得させる力がある。
Her logic is always lucid and has the power to convince those listening.
USAGE:
Most commonly appears in the set phrase 頭脳明晰 (having a sharp mind). Also used with 意識 (consciousness) and 論理 (logic).
FORMALITY:
A formal, literary word. In everyday speech, 頭がいい or 頭が切れる are more common ways to express the same idea.
WRITING:
The kanji 晰 is not commonly encountered outside this word.