«الْحَقِيقَةُ: الْكَلِمَةُ الْمُسْتَعْمَلَةُ فِيمَا وُضِعَتْ لَهُ فِي اصْطِلَاحِ التَّخَاطُبِ»
A literal expression is a word used in the sense assigned to it by the relevant speech convention.
Waḍʿ (assignment) connects an expression with an established meaning. The operative convention determines what counts as literal; the same form may have one literal meaning in ordinary language and another in an established specialized vocabulary.
For example, ṣalāh has a lexical association with supplication, but in legal-religious usage it denotes a particular act of worship. That is a literal technical use within the relevant convention.