Tataki is an earthen floor which is made by mixing "tatakitsuchi" (beaten earth) with lime and water, then beating and solidifying it. It is written as "三和土" because three kinds of materials are mixed to create it.
"Tatakitsuchi" refers to earth made from weathered granite and andesite and which is characteristically solidified by adding lime and water and kneading. Tataki is made by adding lime and water to beaten earth such as Tenkawa-zuchi (Tenkawa earth) in Nagasaki Prefecture, Sanshu-zuchi (Sanshu earth) in Aichi Prefecture and Fukakusa-zuchi (Fukakusa earth) in Kyoto Prefecture, kneading and coating it on the floor, beating and solidifying it, and then leaving it for one or two days before finally washing away the outer surface.
Before the advent of cement, it was originally used to solidify the ground. In modern Japan earthen floors coated with concrete or tiles are also called tataki.