Though it was once a specific type of trance, progressive trance has become a very broad and vague umbrella term that classifies tracks that don't fit into other existing trance subgenres. This is why you get the deep, more atmospheric trance classified as progressive trance, but also big room trance tracks labelled as progressive trance too, even though the difference between them could hardly be any larger.
This confusion is somewhat understandable. There is always a lot of debate about the meaning of 'progressive' when applied to music. Does it refer to the structure or composition of the track, or does it refer to playing with or evolving the tropes of the genre, helping move it forward? For the purposes of this guide, progressive refers to the structure and overall tone of the track.
Progressive trance tracks are slower than other traditional trance genres, usually sitting somewhere between 128-136bpm. Progressive trance productions tend to have a subdued, deeper, more serious tone to them, with more emphasis on atmospherics and evolving sounds and musical motifs.