A group of people that constantly talk about their favourite band, singer, etc.

A series that takes a nation or several by storm selling over a million copies

Pronounced: Fan-dem-ick
The never ending cycle of a fandom's devolution from a respectable group of fans to a group that relies of shipping, fan-fiction, and inappropriate content that eventually leads to the destruction of a fandom.
(JackAttack on Youtube came up with this magical definition)

a fan of your friendship, really likes your friendship and appreciates it

a massive fan of your friendship, enjoys being a friend of yours.