In logic, a logical connective (also called a logical operator, sentential connective, or sentential operator) is a symbol or word used to connect two or more sentences (of either a formal or a natural language) in a grammatically valid way, such that the value of the compound sentence produced depends only on that of the original sentences and on the meaning of the connective. 

The most common logical connectives are binary connectives (also called dyadic connectives) which join two sentences which can be thought of as the function's operands. Also commonly, negation is considered to be a unary connective. 

Logical connectives along with quantifiers are the two main types of logical constants used in formal systems such as propositional logic and predicate logic. Semantics of a logical connective is often, but not always, presented as a truth function. 

A logical connective is similar to but not equivalent to a conditional operator. 

In the grammar of natural languages two sentences may be joined by a grammatical conjunction to form a "grammatically" compound sentence. Some but not all such grammatical conjunctions are truth functions. For example, consider the following sentences: 

The words "and" and "so" are "grammatical" conjunctions joining the sentences (A) and (B) to form the compound sentences (C) and (D). The "and" in (C) is a "logical" connective, since the truth of (C) is completely determined by (A) and (B): it would make no sense to affirm (A) and (B) but deny (C). However, "so" in (D) is not a logical connective, since it would be quite reasonable to affirm (A) and (B) but deny (D): perhaps, after all, Jill went up the hill to fetch a pail of water, not because Jack had gone up the hill at all.
Are grammatical conjunctions truth functions?
. Some but not all such grammatical conjunctions are truth functions.