Just to clear a few things up….
Pagan – They are dead. Pagans, without any prefixes, were followers of pre-Christian religions in Europe. There is no more “pagan” per se, only reconstruction and best efforts. Sometimes modern people refer to modern-day people following these reconstructed or inspired paths as “pagans” – it’s not wrong, just a semantic quibble. I mix up this and Neopagan all the time in casual conversation. But when I need to be specific, this is how I describe ’em.
Neopagan – They are alive – unless they are not. Neopagans follow (to the best of their knowledge and ability) revived or reconstructed spiritual practices from pre-Christian Europe.
Celtic – Celtic is the umbrella term for a number of nomadic cultures that first emerged from what we would now call India, to range across Europe and settle in a lot of it. It is more a term to describe language, culture, and art style than ethnicity. There is no “one, pure” Celtic anything. Never has been, never will be. Very often it is presented to be synonymous with Irish, but that’s far from the whole picture.
Witch/Witchcraft – Wikipedia of all places has an interesting tour of the word “witch“. Its origins are obscure, but for a very long time it has meant things like occult, wise, divination, speaking, and bending/moving. In the present day, witchcraft may be practiced independently of a religious framework.
Wicca/Wiccan – Wicca was invented in the 1940-50s by Gerald Gardner and Doreen Valiente. It takes inspiration from (romanticized) retellings of Medieval and Ancient spirituality. I visit it often, but it isn’t my home.