Alignment
The positioning of elements relative to one another — left, right, center, or justified — to create order and clarity.
In practice
Alignment places elements along shared edges or centers so they read as related and ordered. Text can be aligned left, right, centered or justified, and images and buttons can line up with the edges of text. On a business card, aligning the name, title and phone number on one left edge looks far more deliberate than three items floating at slightly different positions.
Often confused with Grid System
Alignment is lining elements up on shared edges or centers; a grid system is the full framework of columns and rows that supplies those edges.
Try this
Realign every element of a cluttered flyer to just two vertical edges, then compare the before and after versions.