All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The word Tawheed linguistically means oneness or unity. From a Sharee‘ah perspective, Tawheed means what the Kalimat At-Tawheed (Laa Ilaaha illa Allaah) indicates, i.e. negating that there is any god except Allaah and affirming that Allaah is the only God.
Tawheed is of three kinds:
1- Tawheed Ar-Ruboobiyyah (Oneness of Lordship).
2- Tawheed Al-Asmaa’ wa's-Sifaat (Oneness of His names and attributes).
3- Tawheed Al-‘Ibaadah (Oneness of worship).
All of this is detailed in the books of ‘Aqeedah (creed) compiled by Ahlus-Sunnah (the people of the Sunnah).
Kalimat At-Tawheed (The word of monotheism), as mentioned previously, is: Laa Ilaaha illa Allaah (There is none worthy of worship except Allaah).
Tawheed Al-Kalimah (unification of the word) means that Muslims must unify themselves, gather themselves and attune their hearts to major issues and basic principles so that they can establish the entity they aspire to and achieve the goal for which the entire universe was created. To achieve that end, unifying the word must be truly based upon the prerequisites and implications of Kalimat At-Tawheed.
Allaah Knows best.