Tawheed in Arabic means attributing Oneness to Allaah and describing Him as being One and Unique, with no partner or peer in His Essence and Attributes.
The Arabs say waahid, ahad and waheed, all meaning one. Allaah is Waahid, meaning that He has no rivals or peers in any way. So Tawheed means knowing Allaah is One, with none like unto Him. Whoever does not acknowledge Allaah in these terms and does not describe Him as being One with no partner or associate does not believe in Tawheed.