C-command is a relationship in grammatical parse trees which is similar to the idea of "siblings and all their descendents" in family trees.

In the following tree:

• A c-commands C, D, and E.
• B does not c-command any nodes.
• C c-commands A.
• D c-commands E.
• E c-commands D.

   B
  / \
 A   C
    / \
   D   E

A node is said to "dominate" another node if it is above it in the tree (it is a parent, grandparent, etc.) The formal definition is that X c-commands Y if:

• X does not dominate Y.
• Y does not dominate X.
• The first branching node that dominates X also dominates Y.

The branching requirement means that in the following tree, A c-commands D, E, and F.

   C
  / \
 B   D
 |   |\
 A   E F

References

• http://www.criticism.com/linguistics/govt-binding-basics1.php
• Contemporary Linguistics by William O'Grady, Michael Dobrovolsky, and Mark Aronoff. Bedford/St. Martin's. 1997 (third edition).