Reversible & Irreversible Process
The second law of thermodynamics enables us to divide all the processes into two classes:

1. Reversible or Ideal process

2. Irreversible or Natural process

A heat engine is a system operating in a cycle. If a heat engine is reversible, it means the thermodynamic cycle on which it operates has to be
reversible. A thermodynamic cycle consists of more than one non-cyclic process. Hence it is a logical conclusion that for a cycle to be reversible,
each of its processes must be reversible. This makes it necessary to define and understand a reversible process.
A process is said to be reversible, if after the process is completed both system and surroundings may be restored to their initial states without
producing any changes in the rest of the universe.  A reversible process is carried out infinitely slowly with an infinitesimal gradient. So that the
process is quasi-static.
Any natural process carried out with a finite gradient is an irreversible process. All spontaneous processes are irreversible.