Impervius is a spell which makes something waterproof or water repellent.
References from the canon
- Hermione used this spell to make Harry's glasses repel water during a rainy Quidditch match (PA9).
- The entire Gryffindor team used it on their faces to try to practice in a driving rain, but they still gave up after an hour (OP18).
- Recommended by Hermione to Ron as a way of protecting Yaxley's belongings from the rain in his office until it can be stopped (DH12).
- Hermione used this in an attempt to protect herself, Ron, Bogrod, and Griphook from the burning, multiplying treasure in the vault (DH26).
"im-" prefix from L. not + "pervius" L. letting things through