Primer
A surfacing material used to coat a support to get it ready for paint application.
Acrylic primer varies a lot in quality and poor quality products can provide a less absorbent ground than is often preferred. Good quality acrylic primer (can contain upwards of 14 ingredients) is a very good product for both oil painting and acrylic painting. It does both steps of the surface preparation in one- it both sizes (seals) the surface and gives a ground for painting.
Also see Gesso