Any romantic or sexual interaction between female characters. It doesn't matter if it's mutual or one-sided. If it's mutual, consider adding the couple tag.
- For Danbooru tagging purposes, Futanari is not yuri (see forum #11853); use futa with female or futa with futa instead.
- If the bisexual (female) tag applies, use it instead of the yuri tag.
- Wikipedia: Yuri