Used in syntax, a relationship between nodes on a tree. A node X c-commands a node Y if and only if the node that immediately dominates X also dominates Y, and neither X nor Y dominates the other. (Or, put another way, "a node c-commands its sisters and nieces.")

     Z
    / \
   X   Y
      / \
     P   Q

Here,

• X c-commands Y, P, and Q
• Y c-commands X
• P c-commands Q
• Q c-commands P

This relationship is extremely important to theories of government and binding.

See linguistics metanode, Principle A, Principle B, Principle C.