A ‘session’ of Parliament is the period spanning between the first sitting of a House and its prorogation (or dissolution in the case of the Lok Sabha).
A sitting of Parliament can be terminated by adjournment or adjournment sine die or prorogation or dissolution (in case of the Lok Sabha). An adjournment suspends the work in a sitting for a specified period, which may be hours, days or weeks.
The presiding officer declares the House adjourned sine die when the business of a session is completed. Within, next few days, the President issues notification for the prorogation of the session. However, the president can also prorogue the House while in session.
To read more about the Sessions of Parliament, check the linked article.