It's merino. Thin , warm, no sleeves merino. What else is there to say?
Very good. Fits well -- Ornot runs "cyclist" in sizing so on the smaller end of any particular size's range. More fitted than club. More generous than, say, a Santini race cut.
Wonderful, just wonderful. The best product of its kind, in my experience.
I have bought two; one for my husband 6’ 175 # Large and one for myself 5’8 155# Medium. We both agree that the fit is not too tight nor loose, it feels light against the skin and we live in Colorado so it is a go to staple most of the year. We do not wear it when it is temperatures over 85F or below 45F (we wear a little less or more, respectively).