From Urban Dictionary.com (I like this one)
Hot and sexy older woman, usually in her 40s or 50s, single or married, who is sick of her same-age counterparts which are usually hairless, have big guts, who only talk about their insurance premiums and have the TV remote control attached to their hands. Cougars are attractive, in their sexual prime, who know what they want and aren't afraid to go after it. BIG misconception is that they dress cheap, wear hot pink nail polish, animal skin prints and are not-so-attractive old-looking hags with bleached hair (Yeah those women exist, but they are NOT cougars). True cougars are classy, beautiful creatures who have made their successes on their own, have real brains, usually with expensive cars/homes, and are real head turners. Cougars seek younger men, and don't have to sneak up and attack...they know their younger mates are eager to get an experienced woman who won't ask if they'll call them the next day. Being a cougar is a positive thing.
Used for a middle aged woman who hits on twenty-something men.
Cougar Alley Definition (From Urban Dictionary)
A short stretch of bars and restaurants on Rush street (between Oak and Maple) found in Chicago, IL. Found circulating in and around these establishment are numerous cougars. They congregate here in search of nightly prey.