Question

what is meant by collision....

Answer 1

When two or more bodies exert forces on each other for a relatively short time , there exists a event called Collision.

Answer 2

A collision is an event in which two or more bodies exert forces on each other for a relatively short time. Although the most common colloquial use of the word "collision" refers to incidents in which two or more objects collide, the scientific use of the word "collision" implies nothing about the magnitude of the force.
 
example of physical interactions:
An insect touches its antenna to the leaf of a plant. The antenna is said to collide with leaf.