Plain English Meaning
The forces pushing outward on the aircraft skin and windows because the air inside the cabin is at a higher pressure than the air outside.
Full Definition
The mechanical stresses placed on an aircraft's structure -- particularly the fuselage, windows, and windshields -- by the pressure difference between the pressurized cabin interior and the lower-pressure outside air at altitude.
Context Anchor
Seen in discussions of pressurized aircraft windshields, especially when considering windshield strength, heating, icing, cracks, or damage.
Derivation
Pressurization comes from Latin pressura, meaning 'a pressing.' A 'load' in engineering is any force a structure has to carry. So a pressurization load is simply the force the structure must carry because of the pressure difference between inside and outside.