Scientific findings that squirrels are intelligent animals sounded nutty at first, but now that doctors are encouraging people to eat more nuts because they're such healthy food, those squirrels are looking pretty smart. Pigs are considered intelligent too, and they eat garbage.

People and computers are intelligent when they show a high level of understanding. Intelligent comes from the Latin "to understand, to gather," and intelligent creatures gather and make sense of information easily. Many people who aren't labeled intelligent show creativity with resources, or an intelligent use of space, money, or time. While not everyone is considered intelligent — there are even smart vs. dumb computers — most do things that are intelligent, just as intelligent people can do things that are dumb.

Definitions of intelligent

adj having the capacity for thought and reason especially to a high degree

“is there intelligent life in the universe?”
“an intelligent question”
showing mental alertness and calculation and resourcefulness
agile, nimble
mentally quick
apt, clever
mentally quick and resourceful
brainy, brilliant, smart as a whip
having or marked by unusual and impressive intelligence
bright, smart
characterized by quickness and ease in learning
born, innate, natural
being talented through inherited qualities
quick, ready
apprehending and responding with speed and sensitivity
having a keen intellect
brilliantly clever
searching, trenchant
having keenness and forcefulness and penetration in thought, expression, or intellect
characterized by or characteristic of exceptionally early development or maturity (especially in mental aptitude)
stupid, unintelligent
lacking intelligence
lacking or marked by lack of intellectual acuity
brainless, headless
not using intelligence
relatively slow in mental or emotional or physical development
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adj exercising or showing good judgment

“an intelligent solution”
healthy, level-headed, levelheaded, sound
reasonable, sensible
showing reason or sound judgment

adj endowed with the capacity to reason

reasoning, thinking
consistent with or based on or using reason

adj possessing sound knowledge

having or appealing to those having worldly knowledge and refinement and savoir-faire