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An 'ID/LP grammar' is a formal grammar that distinguishes immediate dominance (ID) constraints from linear precedence (LP) constraints. Whereas traditional phrase structure rules incorporate dominance and precedence into a single rule, ID/LP maintains separate rule sets, which need not be processed simultaneously.
For example, a typical phrase structure rule might say S ==> NP VP indicating that an S node dominates an NP node and a VP node, and that the NP precedes the VP in the surface string. In ID/LP grammars that rule would only indicate dominance, and a separate rule indicating linear precedence, such as "[NP,VP]" would also be given.
The idea first came to prominence as part of generalized phrase structure grammar; the ID/LP approach is also used in head-driven phrase structure grammar, lexical functional grammar, and other unification grammars.