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.
Some examples of current and former players in the NBA are Dirk Nowitzki, Draymond Green, Xavier McDaniel, Andre Roberson, James McAdoo, Michael Beasley, Lamar Odom, Josh Smith, Thaddeus Young, Shawn Marion, Antoine Walker, Shareef Abdur-Rahim, Keith Van Horn, Rashard Lewis, Antawn Jamison, Gerald Wallace, Al Harrington, and Kevin Durant.