Thinset
Cement-based adhesive used to bond tile to a substrate. Modified or unmodified, depending on the install.
Thinset is the cement-and-polymer adhesive used to bond tile to a substrate. It's mixed with water (or sometimes a latex additive) on-site and trowelled onto the substrate before the tile is set.
Two main types: unmodified (pure cement and sand, used over membranes that bond mechanically) and modified (with polymer additives, used directly on subfloor). Most uncoupling membrane manufacturers SPECIFY which thinset type their product requires — installing the wrong one voids the warranty.
Always price thinset, grout, and substrate prep as separate line items. Bundling them into "tile install" hides real costs from the customer and from your own job-cost reports.