Productivity:

1. having the power of producing; generative; creative: a productive effort
2. producing readily or abundantly; fertile: a productive vineyard

Productivity is what GTD is all about: creating the necessary tools and attitudes to increase your productivity so as to create and achieve more without a discernible increase in stress and pressure that would otherwise be associated with such increases.

David Allen likes to use the term black belt in regard to GTD. Black belt typically suggests a degree of competence and confidence in abilities to deal with situations. In martial arts for instance this situation could be dealing with an attacker. With productivity, it could be the confidence in oneself to come to terms with a new project quickly (putting it into your GTD system and working out next actions) by being calm and in control of the situation.

Being productive involves knowing why you are doing a project, seeing where you need to go next with it by having next actions, and having the final result in mind at all times. Having this sort of information off your mind allows you to think about the project itself and not the unproductive thoughts of why, how and when.