Acuity: sharpness of thought, vision, or hearing.  The ability to think accurately and clearly.  Penetrating intelligence.

Acuity is insight, perception, sagacity, or astuteness. 

Pronunciation: Acuity

We often use this word in the phrase “mental acuity.”

In idiomatic English, we might say that someone with mental acuity has the “mind of a steel trap.” 

We tend to think that mental acuity declines with age.  This explains why I feel that I have the mind of a dented, rusty steel trap. 

My mind needs some WD-40.