A Light fitting can be classified by an IP rating.
IP stands for Ingress Protection, the degree of protection the fitting has against solids or liquids.
The rating is coded by a two digit number, the first referring to solids, the second to liquids. The first digit is irrelevant for bathroom applications. The second digit code increases with the higher level of protection given.
For example, IP44. The second digit signifies the Light will resist splashing from any direction.
2nd Digit Degree's of Protection 0 non protected
1 protected against dripping water
2 protected against dripping water when tilted up to 15 degrees
3 protected against spraying water
4 protected against splashing water
5 protected against water jets
6 protected against heavy seas
7 protected against effects of immersion
8 protected against effects of submersion
Lights suitable for installation in most areas of the bathroom are IPX4, usually IP44 and above.
For full details refer to IEE Wiring Regulations (16th Edition)
For installation guidance the bathroom is split into Zones. The IP rating of the light dictates in which zone it may be fitted.