Known in the modern era as “intention–manifestation”, magick is a paranormal power that allows you to manifest your desires in reality, usually performed via a ritual (or “spell”) wherein the desired outcome is visualized. The ritual may contain other actions such as concentration meditation, the saying of magickal words, the creation of sigils, or other activities according to the magickal system being used at the time.
I use the old spelling “magick” here to distinguish these activities from the other kind of “magic” (sleight of hand, illusion, and other performance tricks which are nothing to do with what we are talking about here).
People who practise magick are usually referred to as “occultists”, and some well-known ones are Aleister Crowley and Peter J. Caroll.
Magick is a type of siddhi. Magick and siddhis are usually banned in major religions and are referred to by names such as “Satanism”. This is because major religions teach terror at the unknown and don’t know what they are talking about.