A-position is a position that in D-structure can be occupied by an argument. A-positions are positions to which a theta-role can be assigned (subject and object positions). A-positions are also known as Argument positions. A position which is not an A-position is called an A'-position (A-bar-position).
The NPs John and apples in the sentence John eats apples are in A-positions (in D-structure).
The position occupied by operators such as who in e.g. who does he see? is an A'-position. Another term for A'-position is Non-Argument position.