An international rating system for the degree to which an object is protected against water, dust, and other outside material.
A full IP rating consists of the letters "IP" followed by two digits. The first digit is for dust and other solids. The second digit is for liquids. An "X" rating is blank, so for example "IPX7" means "not rated" or "not listed" for dust and rated 7 for liquid. Higher digits are better.
For dust, ratings 4 and below only cover relatively large objects. A rating of 5 is protected against dust, while 6 (the highest for solids) is dust-proof.
For liquids, the range is from 1 (only protected against drips from above) to 9 (can survive a dishwasher or other high-pressure, high-temperature spray-down.)
Common liquid ratings include:
IPX4: Protected from splashes from any angle.
IPX5/IPX6: Protected from water jets from any angle. This is common for phones labeled "water-resistant".
IPX7: Can be submerged in water up to 1 meter deep for up to 30 minutes. This is common for phones labeled "water-proof".
IPX8: Can be submerged in deeper water for longer periods of time.