A process that proceeds on its own without any outside assistance.

Energy is conserved.

In general, processes that are spontaneous in one direction are nonspontaneous in the opposite direction

Reversible pocess: change occurs in such a way that the system and the surroundings can be restored to their original states by exactly reversing the change.
Ex: transfer of heat when both system and surroundings are at the same temperature

A process that cannot simply be reversed to restore the system to its original condition with no net change to either the system or its surroundings.
Ex: transfer of heat when both system and surroundings are at different temperatures

A process that occurs at a constant temperature.