Tawhid is an Arabic number derived from the Arabic number one. One is translated to wahid in Arabic. English translation of tawhid is placed under theology. It could also mean unite or join according to the context of the sentence, but basically, it all goes back to the number one.

Variation examples of the number one and how it could be used for unity,

Al Welayat Al Motahida Al Amreikyah. -- United States of America.
Qol howa Allah ahad. -- Say god is one.
Fel itihad qowa. -- There is strength in unity.

Tathleeth on the other hand is derived from the Arabic number thalatha. Tathleeth is trinity.