PRIMITIVE
(A form (or expression) is a conventional pairing of a shape, i.e. a continuous sequence of segments, and a content, i.e. meaning or discourse function or grammatical function.)
Instead of "a form", one may also say "an expression". These terms refer to the same concept, but longer forms (such as nominal phrases or clauses) are usually called "expressions".
In Croft (2022), the term "element" is used roughly in this sense.
Note that "form" is often used alternatively for the signans of a linguistic sign, and signs are often said to have a "meaning" side and a "form" side. For this latter sense, it is better to use "shape" if one wants to be precise, because a form/expression (in the sense of the current entry) is itself a sign.
Note that the shapes of forms/expressions must be continuous. Parts of forms/expressions that belong together but are not continuous are not forms/expressions, but something else (for example, the plural suffixes -se and -s in the-se book-s belong together, via concord, but they cannot be treated as a single expression because they are not continuous).
element (CXN) = a word or construction that is part of – that is, functions in a role in – a construction. Example: in an ancient watch, the elements are an, ancient, and watch. (Section 2.2.1)
“Every language consists of a number of signals, linguistic forms. Each linguistic form is a fixed combination of signaling-units, the phonemes . . . We assume that each linguistic form has a constant and definite meaning.” (Bloomfield 1933: 158)