A Full Quad or Unicorn Quad is a polyamorous relationship between four people where all four people are dating each other. This can be considered a mix of a group relationship and a quad relationship.
It is unknown when the term was first coined however it is known to have been around since 2018.
The flag was coined by Tumblr user beyond-mogai-pride-flags on November 17th, 2018. It has no confirmed meaning.