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. 

Further Reading:

• Lok Sabha Speaker [Article 93 – 96]
• President of India – Article 52-62

To read more about the Sessions of Parliament, check the linked article.