Definition - What does Tathata mean?
Tathata is a word in Sanskrit and Pali that is mainly used in Mahayana Buddhism to refer to the true nature of reality. It also has a significance in Chan Buddhism. Tathata is often translated as “suchness” or “thusness,” and it is widely accepted that the true nature of reality is inexpressible in language because it is beyond both description and conceptualization. It can be used at the end of a discussion of a particular teaching to indicate that the teaching is ineffable.
Tathata only ever refers to what is right now, as it means the “suchness” of the moment.
Yogapedia explains Tathata
Some teachings suggest that tathata is what underlies reality and, therefore, things that appear in the physical world are all manifestations of tathata. In this way, tathata may be used interchangeably with sunyata, which means "emptiness," because all things are seen as being empty of any self-essence, but, simultaneously, they are also full of reality itself. Another synonym which is sometimes used is dharmata.
The Buddha referred to himself as the Tathagata, which means “One who has arrived at suchness.” In this way, the term indicates that Buddha had arrived at and awakened to the true nature of his reality.