Netorare (寝取られ) are games where someone steals the heroine away from the protagonist.
These types of games are designed to cause the player to have sexual jealousy.
For a game to be classified as Netorare the following conditions must be met:
1. The heroine must be close (osananajimi, etc.), romantically involved or a close relative (sister, mother, etc.) with the protagonist.
2. If it is an eroge, related sexual events are explicitly shown to the player or later described in detail.