In game theory, a subgame is any part (a subset) of an extensive form game that meets the following criteria:

1. It has a single initial node that is the only member of that node's information set (i.e. the initial node is in a singleton information set).
2. It contains all the nodes that are successors of the initial node.
3. It contains all the nodes that are successors of any node it contains.
4. If a node in a particular information set is in the subgame then all members of that information set belong to the subgame.

It is a notion used in the solution concept of subgame perfect Nash equilibrium, a refinement of the Nash equilibrium that eliminates noncredible threats .