noun
intellectual power; mental capacity; intellect; intelligence
Refers to the faculty of reasoning, understanding, and acquiring knowledge; often used in contexts of mental ability or brainpower.
彼は知力に優れている。
He excels in intellectual power.
知力を鍛えるには読書が効果的だ。
Reading is effective for training the intellect.
知能 is closer to 'intelligence' as a measurable cognitive ability (e.g., IQ), while 知力 emphasizes the power or capacity of the mind more broadly.
頭脳 literally means 'brain' and often refers to intellectual ability in a more concrete or metaphorical sense (e.g., 'brains of the operation'), whereas 知力 is more abstract.
Compound of 知 (chi, 'knowledge, wisdom') and 力 (ryoku, 'power, strength'). The spelling 智力 uses the older character 智 for 'wisdom', but 知力 is now standard.