Thinset mortar is a mixture of Portland cement and a clean, fine-grain, sand. It usually also contains a water-absorbent material to adjust the setting speed and other characteristics. The name describes how the mortar is applied.

Thinset is used to bond ceramic tile to a surface. It is applied in a thin layer whose purpose is simply to cement the tile to what is behind it, which may be a floor, a table surface, a wall or a ceiling among other things. The final mortar layer may be as little as 3/16 of an inch (5 mm) thick. To get a uniform layer of that thickness, the thinset is applied with a trowel that has square notches that are 3/8 of an inch (10 cm) wide. Such a trowel leaves long, square, parallel ridges of mortar on the surface (substrate). The final thin layer is formed when the tiles are pressed into place.

Thinset is fast, easy and cheap to use because a minimum amount of mortar is required. Thinset mortar will not work well if the surface you want to tile is not flat, because the finished tile will have about the same surface curvature as the substrate. To level off depressions and prominences, or to achieve a purposefully curved area such as for a floor drain, you may have to lay down a preparatory medium or thick layer of mortar. When that layer sets, you can then use thinset to cement the tile to it in the usual way.

Nowadays, you can buy what is called latex thinset, which is modified with combinations of a huge number of polymer additives, many of which are not latex at all. The clever manufacturers of thinset mortar can choose the polymers so as to give the mortar different characteristics, including flexibility, resistance to freezing, and waterproofing.

Now you know what’s under that tile you might be looking at. The observant will note that the gaps between the individual tiles are filled with something. That something is called grout, which is a bit different from mortar.