The Notional Syllabuses (1972) is a proposal for the teaching of language created by D.A. Wilkins, and also proposed by Van Ek and Alexander in 1975. The full name of the proposal is the Notional-Functional syllabus and it consists on arranging language lessons by notions (main ideas), and functions, which is the name given to the vocabulary that is produced as a result of that "big idea". This system was an answer to the Audio-Lingual Method of teaching languages, which was based on listening and repeating words and other utterances. As part of the Communicative Language approach, the Notional Functional Syllabus was highly regarded as a best practice.
Examples of Wilkins's definition of communication can be illustrated the following way:
Ex: notional category= quantity
Communicative functions of the notion- Asking how many are there, stating amounts, buying and selling a number of things.
As you can see, out of one "notion", or main idea, we can formulate a number of communicative functions, which are just the same as potential vocabulary application.
Another notional category could be time
Communicative functions of the notion - Telling time, early or late, soon or later, "when is?" questions, "at what time" questions, and anything related to that topic.