Term definition and assimilation is a significant part of education. The definitions offered in didactic contexts tend to oscillate between lexicographic rigour and the didactic need to explain, to exemplify, to classify inherently, and to codify graphically; student age is also a factor. Textbook definitions lean visibly towards the encyclopaedic model, although a textbook lacks the space for either the full text of an encyclopaedia definition or the appropriate number of illustrations. It is not the learning by heart and verbal reproduction of definitions that is truly essential, but the skill of constructing definitions: what they should contain and what they ought to look like.