. One's nearness in time, relation, space, occurrence, order.
. Groups related items together.

Law of Proximity:

. Spatial or temporal proximity of elements may induce the mind to perceive a collective or totality.

Real Life Examples : 

Arranging books and bags

Items in proximity

A spoon holder

Proximity in design:

. Related design elements must be placed together.
. Separate information in logical order.
. Use appropriate fonts.
. Create visual hierarchy.
. Use grids.