A “hyperfixation” is the complete obsession or intense and prolonged focus over a particular hobby or task. This could be a special interest, a TV series, a new subject you are researching, a new relationship even.
As an AuDHDer my hyperfixations tend to be centred around my special interests of music and reading and tend to last around a few weeks but they can also be totally new hobbies I get into for up to a year and then leave, never to touch them again.