What is "c-command"

n. (context syntax English) The relationship between a node in a parse tree and its sibling nodes (usually meaning the children of the first branching node that dominates the node) and all the sibling nodes' children. vb. (cx syntax transitive English) To dominate in a c-command relationship.

c-command (constituent command) is a relationship between the nodes of grammatical parse trees. It is closely associated with the phrase structure grammars of the Chomskyan tradition ( Government and Binding, Minimalist Program), and may not be valid or applicable to the tree structures of other theories of syntax, such as dependency grammars. The relation of c-command has served as the basis for many explorations and explanations of phenomena of syntax. It has been taken to be the basic configurational relation underlying binding, and has played a central role in the analysis of diverse syntactic mechanisms, such as parasitic gaps and the scope of quantifiers.

Informally speaking, a node in a tree c-commands its sibling node(s) and all of its siblings' descendants; however, a node without siblings c-commands everything that its parent c-commands.