|Table of contents||Table of diagrams
fewest number of plays
greater number of plays
territory efficiency 0.67
territory efficiency 1
territory efficiency 1.83
inefficient: too slow movement
efficient: good compromise between speed and safety
inefficient: too insecure
A player's greater efficiency achieves a) something with the fewest number of his plays or b) the most with the same number of his plays. 
Note that plays made for then removed stones are also counted.
Example 1 for different numbers of black plays resulting in different, compared positions: Suppose the task is to create a solid black wall in front of the white stones.
Black achieves the task with 7 plays. This is inefficient, because Black spends one play too many, which he would better play elsewhere.
Example 2 for same number of black plays: Suppose the task is to live while making the most territory. Each diagram needs 6 black stones played.
Black has played 6 stones to get life with at most 3 points. This is inefficient.
Black has 4 points made by 6 stones played. His territory efficiency is 4/6 ~= 0.67. This is inefficient.
Black has 6 points made by 6 stones played. His territory efficiency is 6/6 = 1. This is better than in Example 3. In relation, Black has made territory more efficiently.
Black has 11 points made by 6 stones played. His territory efficiency is 11/6 ~= 1.83. This is more efficient than in Examples 3 and 4.
Efficiency is the optimal compromise between safety and speed of local movement. 
This more specialised description of efficiency has, however, its merits for related application.
 Citation from Joseki Volume 2 Strategy. In Fundamental Principles of Go, published 2004, Yang Yilun appears to have been the first to relate number of moves and amount of territory. I have clarified division to be the kind of relation.