The French phrase means sentence while the English “phrase” can be translated by expression, proposition, tournure, énoncé and various other words, depending on the context.

You might then write:

Définir le système, la frontière, la frontière adiabatique, la frontière diathermane, l’environnement, la chaleur, les variables d’état et l’équation d’état en quelques mots ou une phrase complète.

If you are talking about English, in linguistics a phrase is a part of a sentence, which can amongst others be used for answering. So in :”When are you coming?” “In winter/at 3 pm”, the latter are prepositional phrases. In French the linguistic term is “syntagme”.