May I know the difference between lawsuit and litigation?
Any lawyers and eng teachers would like to help?

According to http://www.translegal.com / ,

a legal problem that does not involve a crime brought before a court of lawOur first consideration is how to avoid a lawsuit.

the process of taking a case to a court of law so that an official decision can be made; legal proceedings that may lead to a full trialBecause the defendant refused all attempts to reach a settlement, the claimant had no choice but to continue the litigation.

So can they be used interchangeably?

thank you

They are different. A lawsuit is an instance of a civil (not criminal) legal dispute between a plaintiff and defendent.
Litigation is the process that the lawsuit goes through after it has been formally filed. Litigation can be very expensive, since it involves legal council, gathering evidence, depositions, fiings with the court, etc.

In the US, the process is subject to state, not federal laws. You can read about the process in California HERE.