In the terms of Relation and Functions, Reflectivity is understood as the identity relation.

A relation is said to be reflexive, if and only if for the given set, for each and every elements it's reflection i.e., doublets must be present.

For more details and explanation, click on the link below:

Oneshot: https://www.youtube.com/watch?v=805ElUjEJ1I&t=15s

Revision: https://www.youtube.com/watch?v=4tpvHOjsw94&t=5573s

JEE:  https://www.youtube.com/playlist?list=PLCzaIJYXP5YdFtkM5hQdxBDnK8xo3Ps1V