410.10: Luminaires in Specific Locations.

(A) Wet and Damp Locations. Luminaires installed in wet or damp locations shall be installed such that water cannot enter or accumulate in wiring compartments, lamp holders, or other electrical parts. All luminaires installed in wet locations shall be marked, “Suitable for Wet Locations”. All luminaires installed in damp locations shall be marked, “Suitable for Wet Locations” or “Suitable for Damp Locations”.

From the 2017 National Electrical Code   Underwriters Laboratories (UL) Definition
Dry Locations:

A location not normally subjected to dampness or wetness. A location classified as dry may be temporarily subject to dampness or wetness, as in the case of a building under construction.

  Dry Locations: A location not normally subject to dampness, but may include a location subject to temporary dampness, as in the case of a building under construction, provided ventilation is adequate to prevent an accumulation of moisture.

Damp Locations:

Locations protected from weather and not subject to saturation with water or other liquids but subject to moderate degrees of moisture. Examples of such locations include partially protected locations under canopies, marquees, roofed pen porches, and like locations, and interior locations, subject to moderate degrees of moisture, such as basements, some barns, and some cold storage buildings.

 

Damp Locations:

An exterior or interior locations that is normally or periodically subject to condensation of moisture in, on, or adjacent to, electrical equipment, and includes partially protected locations.

Wet Locations: Installations underground or in concrete slabs or masonry in direct contact with the earth; in locations subject to saturation with water or other liquids, such as vehicle washing areas; and in unprotected locations exposed to weather.   Wet Locations: 

A locations in which water or other liquid can drip, splash, or flow on or against electrical equipment. A wet location luminaire shall be constructed to prevent the accumulation of water on live parts, electrical components, or conductors not identified for use in contact with water.

A luminaire that permits water to enter the luminaire (during the rain test of Clause 16.5.2 or the sprinkler test of Clause 16.5.3) shall be provided with a drain hole (Clause 13.4.3).