A tweener in basketball is a term, sometimes used derisively, for a player who is able to play two positions, but is not ideally suited to play either position exclusively, so he/she is said to be in between. A tweener has a set of skills that do not match the traditional position of his physical stature.
NBA.com's definition of "tweener" is as follows:
A player who is ideally suited to play two positions is sometimes referred to as a swingman, although that term is more commonly reserved specifically for those who are suited to play small forward and shooting guard.
This tweener has the skills of either a center or a power forward, but is usually stronger than traditional power forwards and quicker and often more skilled than traditional centers, and is generically called a "big" in American basketball, where the distinction between power forwards and centers has become increasingly blurred. Many times C/PF tweeners are used to create match-up problems. Amar'e Stoudemire is an example of a tweener. Other prominent NBA players who switch between power forward and center are Tim Duncan, Kevin Garnett, Pau Gasol, Chris Bosh, LaMarcus Aldridge and Anthony Davis, among others. A good example of such a Euroleague player is Mike Batiste.
Traditionally, a SF/PF tweener refers to a basketball player whose physical attributes and skills render him/her unsuited to play either the small forward position or the power forward position exclusively. For example, the player may be too short, not athletic enough, or perhaps lacking proper ball handling/shooting skills to play small forward; while lacking sufficient strength to play the power forward position effectively. On the other hand, the player may have the skills to play either forward position, but does not necessarily fit either of them exclusively. They can be too big for most opposing small forwards to guard them and have a skill set that small forwards traditionally have (ex. outside scoring ability).