As a Noun: 1. The condition of having lost one's mental powers and become less and less capable, especially because of age. English = senility. As a Verb or Adjective: 1. To lose one's mental powers and become less and less capable, especially because of age. English = become senile, be senile. 2. Of mental capacity or intelligence or knowledge, to become diminished, less and less, weaker or enfeebled. 3. Of a person's mental capacity or intelligence, to be very low, weak or impaired. English = unintelligent.