The strip under a ski or splitboard whose hairs grip when you slide back and lie flat when you glide forward. Mohair/nylon blends (often 70:30) are the standard.
A climbing skin is directional: the hairs lie flat as you glide forward and stand up the moment the ski wants to slide back. Pure mohair glides best and wears out fastest, nylon grips and lasts longer — which is why the blend, often 70:30, is the usual compromise. The typical beginner mistake is expecting a skin to hold any gradient: past about 20 degrees of track angle grip drops off noticeably, and that calls for a zigzag track rather than brute force. Past roughly 25 degrees pure skinning becomes inefficient — that's where ski crampons or bootpacking come in. What often gets missed: the glue is temperature-sensitive. In the cold, fold skins glue-to-glue and carry them against your body, or you'll reach the turnaround with two useless strips.