Tauhid in Arabic is a masdar form of wahhada-yuwahhidu (with the letter ha in tashdid), which means: to make something only one thing. So, the definition of tauhid is offering worship only to Allah.
And this definition can only be applied truly if someone denies everything that worshiped besides Allah. Noble Shaykh Muhammad ibn Salih Al Othaimin said: “The meaning of tauhid is still incorrect unless it is followed by a disclaimer. That is to deny everything other than something that we just make one, then just set it” (Sharh Thalathatil Usul : 39).
In the other term, tauhid is to make Allah the only true deity with all its specificities (Sharh Thalathatil Usul : 39).
From these explanations, it can actually be understood that many things in this world are worshiped by humans, it can be in the form of angels, prophets, righteous people or even other creatures of Allah, but when someone practicing tauhid he only worships Allah and denies everything worshiped beside Allah.
In surah Al Fatihah, that we always recite everyday:
إِيَّاكَ نَعْبُدُ وَإِيَّاكَ نَسْتَعِينُ
“It is You we worship and You we ask for help” (Al Fatihah: 5).
Allah said in the Qur’an:
وَلَقَدْ بَعَثْنَا فِي كُلِّ أُمَّةٍ رَسُولًا أَنِ اعْبُدُوا اللَّهَ وَاجْتَنِبُوا الطَّاغُوتَ
“And We certainly sent into every nation a messenger, [saying], “Worship Allah and avoid Taghut.”” (An Nahl: 36).
Alla also said:
ذَلِكَ بِأَنَّ اللَّهَ هُوَ الْحَقُّ وَأَنَّ مَا يَدْعُونَ مِنْ دُونِهِ هُوَ الْبَاطِلُ وَأَنَّ اللَّهَ هُوَ الْعَلِيُّ الْكَبِيرُ
“That is because Allah is the true God, and that which they call upon other than Him is falsehood, and because Allah is the Most High, the Grand.” (Al Hajj: 62).
So, worshiping only Allah and denying all that worshiped besides Allah, that is the meaning of tauhid.