Things it is not, or not obviously
Sometimes bead lightning is mistaken for ball lightning. Britannica Bead lightning is most apparent when the current in a cloud-to-ground flash persists for an appreciable fraction of a second. In those cases the luminosity also persists and the channel may have regions of enhanced luminosity that resemble a string of beads. Britannica
The Met Office also lists pearl-necklace lightning, also termed chain lightning or beaded lightning, in which variations of brightness along the discharge path give rise to a momentary appearance similar to pearls on a string. Met Office Rocket lightning, in the Met Office list, is a very rare and unexplained form in which the speed of propagation of the lightning strike is slow enough to be perceptible to the eye, normally seen at the top of the thunderclouds. Met Office
Ordinary fork lightning, the RMetS article notes, is an atmospheric electrical discharge, typically formed when a negatively charged leader from a cloud is met by a positively charged streamer. RMetS
One explanation offered by sceptics, the article says, is that ball lightning is merely a positive after-image left on the eye in the wake of a standard lightning flash. RMetS
Dr Lindsay said, in the article’s wording, “I don’t think what we’ve created is lightning,” and that ball lightning is used almost generically for phenomena not described by normal lightning, bead lightning, St Elmo’s fire or aurora. RMetS
The RMetS article says some scientists have studied ball lightning for decades and still are not sure whether it exists. RMetS
Keul also notes a COST Action proposal by Thottapillil (2005), with 30 European researchers, and the consensus that “the reality of ball lightning is not in doubt.” Keul 2021
Are scientists sure it exists?
The 2013 Royal Meteorological Society article says some scientists have studied ball lightning for decades and still are not sure whether it exists, and records the sceptical suggestion that it may be a positive after-image of an ordinary flash. Keul (2021) notes a 2005 COST Action consensus that “the reality of ball lightning is not in doubt,” and quotes Rakov and Uman on numerous witness reports but little scientific documentation — before the 2012 spectrum was published. The 2014 paper reports one well-instrumented natural event. This site reports those positions together and does not settle the argument.
Bead lightning, rocket lightning and St Elmo's fire are named separately in the sources used here. The after-image idea is a sceptical explanation for some reports, not a proof that every report is an illusion. The 2012 spectrum is one reason later papers treat at least some events as physical.