Plaster/ˈplæstər/ Definition 1 of 5Pro Tip 1/2Building Material Plaster, in construction, refers to the mixture spread on walls and ceilings. The builders used plaster to finish the interior rooms. Pro Tip 2/2Surface Cracks Plaster is prone to cracking as buildings settle, requiring upkeep. Cracks have appeared in the plaster, so it needs repairing.