Synesthesia is a neurological sign. Involuntarily and constantly, one stimulus evokes another feeling. (Types of synesthesia: thesynesthesiatree.com)

This is a group in Reddit — reddit.com/r/Synesthesia

4 criteria of synesthesia

1. Presence since childhood.
2. Consistency of the sensations, they should always be the same.
3. Vividness (sensory-like quality) of the sensations.
4. Automatic and involuntary occurrence.