Full Idea
The logical forms of the sentences in a language are those aspects of their meanings that determine the relations of deductive entailment holding amongst them.
Clarification
An 'entailment' is a valid step in logical reasoning
Gist of Idea
Logical form is the aspects of meaning that determine logical entailments
Source
Paul Horwich (Truth (2nd edn) [1990], Ch.6.30)
Book Reference
Horwich,Paul: 'Truth (2nd edn)' [OUP 1998], p.88
A Reaction
A helpful definition. Not all sentences, therefore, need to have a 'logical form'. Is the logical form the same as the underlying proposition. The two must converge, given that propositions lack the ambiguity that is often found in sentences.