I have a fairly liberal definition of "sleeper." I think it's any player that is being projected much lower than they should be. For me, DeAndre Hopkins is a sleeper. John Simon is a sleeper. Jordan Hill is a sleeper. The term sleeper implies a player that you probably haven't heard of before. But at the end of the day, the criminally under-rated player you've heard of is no less of a bargain.
Those sites tend to focus on players from division I schools that aren't being talked about a ton. Players like Korey Toomer last year. I liked the Toomer pick, but I never really thought of him as a sleeper. Actually, maybe for the Seahawks he was. But most late round picks that turn into great players do so because they went to the right team at the right time. To a coaching staff that knew how to get the most out of them. Both of our 7th rounders last year illustrated that perfectly.