A--- It means associating partners with the Lord of the worlds, like loving someone in the way one loves Allah, fearing someone as if he is Allah, relying on him, invoking him, hoping from him, sinful loyalty to him, obeying him in what displeases Allah, etc. Allah, the Almighty says:

{Verily! Allah forgives not (the sin of) setting up partners (in worship) with Him, but He forgives whom He wills sins other than that, and whoever sets up partners in worship with Allah, has indeed strayed far away}. (An-Nisa', 116)

Allah the Almighty says: {... and whoever sets up partners with Allah in worship, he has indeed invented a tremendous sin}. (AnNisa', 48)

Allah the Almighty says: {Verily, whosoever sets up partners (in worship) with Allah, then Allah has forbidden Paradise to him, and the Fire will be his abode}. (Al-Ma'idah, 72)

Allah the Almighty says: {..and whoever assigns partners to Allah, it is as if he had fallen from the sky, and the birds had snatched him, or the wind had thrown him to a far off place}. (Al-Hajj, 4)

The Prophet (Peace be upon him) says: "What is due to Allah from His creatures is to serve Him alone and never to associate any other being with Him. What is due them is not to punish any person who does not associate aught with Him.” (Bukhari and Muslim)

There is no difference between those who declare their polytheism like the Pagans of Quraish and those who hide it, like the hypocrites who declare Islam and make their internal polytheism secret. Allah the Almighty says: {Verily, the hypocrites will be in the lowest depth (grade) of the Fire,. no helper will you find for them. Except those who repent (from hypocrisy), do righteous good deeds, hold fast to Allah, and purify their religion for Allah, then they will be with the believers}. (An-Nisa', 145,146)