The National Electrical Manufacturers Association (NEMA) defines standards used in North America for various grades of electrical enclosures typically used in industrial applications.
Each is rated to protect against personal access to hazardous parts, and additional type-dependent designated environmental conditions.
A typical NEMA enclosure might be rated to provide protection against environmental hazards such as water, dust, oil or coolant or atmospheres containing corrosive agents such as acetylene or gasoline.
[1] Below is a list of NEMA enclosure types; these types are further defined in NEMA 250- Enclosures for Electrical Equipment.
Note that higher numbers do not include the lower-numbered tests.