A base layer or thermal should be your first layer when skiing, this important layer wicks away any moisturise generated when skiing and transports it out through your breathable layers, including your ski jacket.
Base layers and thermal layers both have moisture-wicking properties. As seen in the video, a moisture-wicking fabric quickly wicks, or moves, sweat away from your skin, pulling the moisture through the fabric until it reaches the outer fabric on the ski jacket where it evaporates.
Moisture-wicking keeps your skin dry, allowing you to feel more warm and comfortable than you would if you wore damp layers of clothing which can cause sweaty chills, particularly on ski chair lifts.
Thermals are a type of base layer, but not all base layers are thermals.
Both types provide moisture-wicking, but the key difference is that thermal fabrics provide more warmth. Remember the purpose of a moisture-wicking base layer is to remove sweat, to keep you dry and prevent chills, not add warmth.
A Ski Thermal fits snugly over your body, trapping heat against your skin and keeping you warm (while wicking the moisture away). You can find different weights of Thermals which provide varying degrees of warmth which you do not find in a base layer.
Icebreaker offer 200g and 260g of merino insulation in their popular Oasis Sking Thermals and Helly Hansen offer a choice between a LIFA Thermal and a LIFA Merino Thermal which adds in an extra layer of merino for warmth.
So you need to decide on the various factors, early or late skiing? high or low resorts? America or Europe? If skiing in sunny Europe in late spring a simple base layer is ideal, to primarily wick moisture away and keep you dry. Skiing early season in a high resort you would probably opt for a heavier weight Skiing Thermal and a Fleece Midlayer which you can adapt according to that days weather forecast.