Wool-synthetic blends combine merino wool with polyester or nylon to give you the best of both worlds. You get merino's natural odor resistance so you can wear the base layer multiple days without it getting funky, plus the added durability of synthetic fibers that hold up better to frequent washing and the friction of layering under ski gear.
These blends are an excellent choice if you ski both resort and backcountry and want one versatile base layer. They handle all-day lift sessions and skintrack laps equally well, and the synthetic component means they won't break down as quickly as pure merino under backpack straps or tight harnesses.
The trade-off is you lose a bit of merino's plush, natural feel against your skin, and moisture absorption isn't quite as good as 100% wool. But for most skiers and riders, the durability boost is worth that small compromise — pure merino can develop holes relatively quickly with regular use, and blends help solve that problem.