Barclay observing phased matter in the transporter beam
Phased matter was the term used to describe material in a transporter beam as it was being converted into energy.
An individual being transported could see phased matter all around them as they were in the beam. In 2369, Lieutenant Barclay was able to perceive members of the crew of the science vessel Yosemite, who had become trapped in the beam. (TNG: "Realm Of Fear")