Entropy change of system in adiabatic irreversible process In an adiabatic irreversible process we know that q = 0. Now change in entropy is defined as q/T. So when q is zero, why is change in entropy not zero ?
The change in entropy is the reversible heat flow divided by $T$.  So if the process is irreversible, you can't use the irreversible process's heat flow to find the entropy change.