The Ingress Protection Rating, also referred to as IP code or IP rating is an international standard which indicates the sealing effectiveness of mechanical housings against water and dust. The IP rating consists of "IP" followed by two digits, for example IP65. The first digit indicates solid particle protection and the second digit protection against liquid ingress. A higher digit denotes higher level of protection.
The IP rating is published by the International Electrotechnical Commission (IEC) under the international standard IEC 60529.
Further reading: IEC IP Ratings