thermal condcution

Not defined means? In what sense?

according to him, when heat conduction starts, the heat current is not constant and varies till it reaches steady state. After steady state is reached, the heat current remains constant (for a particular cross-section) hence in steady state both dQ/dt and deltaQ/delta t are defined but in transient state dQ/dt is not defined.

Is this true?? I mean how can current vary for a particular cross-section?