Activity-Goal Alignment (AGA) is a way for you, the learning game designer to say how a game's play activity is aligned with its learning goal. Learning games have two important traits: 1) they seek to impart something (the learning goal), and 2) they offer some kind of interaction or mechanic (activity) to players.
Just as rule of thirds allow artists to share tacit composition knowledge, AGA names a slippery but important quality of learning games. Most designers tacitly know the relationship between activity and goal is important, but lack a term to describe it.
One can "score" or rate the relation between an activity and a goal. For example, a driving simulator gets a high AGA score because the activity (simulated driving) and the learning goal (real-life driving) are similar. However, a driving e-learning application uses a puzzle mechanic: choose the answer from a list. This is clearly not similar to driving a car, so it has a low AGA score. Note that both are useful for learning: Parking rules may be better explained in text, while passing safely may be best learned from a simulator.
AGA Score is a "rule of thumb", not a scientific measurement. While a numeric value can represent an AGA Score, it is often better to say "high" or "low".
Complex learning games often have more than one learning goal and activity. AGA is most meaningful for one goal and one activity.
For example, GameStar Mechanic seeks to teach both high-order skills such as systems thinking, and practical skills around game design. It uses many activities and mechanics to do so.
The term "activity-goal alignment" was coined and described by Brett Shelton (http://works.bepress.com/brett_shelton). Shelton and Scoresby define AGA and gives a case study of how AGA affects learning game design in http://digitalcommons.usu.edu/itls_facpub/115/.
AGA summarizes a family of related learning game design principles. A good summary of the key ideas is given on pages 25-38 of http://education.mit.edu/papers/MovingLearningGamesForward_EdArcade.pdf.
For a general introduction to learning games, see J.P. Gee's book "What Video Games Have to Teach Us about Learning and Literacy" available at Amazon. His personal site http://www.jamespaulgee.com includes useful essays