The term `closure` in the context of the conduct of government business in the Parliament of India refers to a rule of legislative procedure. This rule allows for the halting of further debate on a motion.

Option 1, suspension of debate at the termination of a day`s sitting of the Parliament, is not the correct definition of closure. Suspension of debate at the end of a day`s sitting is known as `adjournment`, not closure.

Option 3, the termination of a parliamentary session, is also not the correct definition of closure. Although closure can potentially lead to the termination of a parliamentary session if all necessary business has been concluded, closure itself does not refer to the termination of a session.

Option 4, refusal on the part of the government to have the opposition look at important documents, is unrelated to the term `closure`. Closure specifically refers to the halting of further debate on a motion, not document refusal.

Therefore, the correct answer is option 2, a rule of legislative procedure under which further debate on a motion can be halted.