Order condition

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The order condition is the state of a simultaneous equation in an econometric system such that its parameters may all be identified.

Specifically the order condition states that:

For an equation in a system of equations to be identified, the number of excluded exogenous variables in that equation must be at least as great as the number of included endogenous variables, less one.

However, a stronger argument is the rank condition which is both necessary and sufficient for identification.