Ilhad literally means to turn away. In this regards, its turning away from the truth of the Asmaa wa Sifaat (Names and Attributes) of Allah (Subhanahu wa Ta’ala).
Allah (swt) says,
And you were not covering yourselves, lest your hearing testify against you or your sight or your skins, but you assumed that Allah does not know much of what you do. And that was your assumption which you assumed about your Lord. It has brought you to ruin, and you have become among the losers.â€So [even] if they are patient, the Fire is a residence for them; and if they ask to appease [Allah], they will not be of those who are allowed to appease. [41:22-24]
The emphasis in these ayaat is their denying the attribute of Knowledge from Allah (Subhanahu wa Ta’ala).
And,
And to Allah belong the best names, so invoke Him by them. And leave [the company of] those who practice deviation [ilhad] concerning His names. They will be recompensed for what they have been doing. [7:180]
Examples of ilhad include to deny or distort their meanings, or claim that they have no meaning (tafwid). Considering His Attributes to be similar to the creation, deriving Names not befitting Allah (e.g. “The Father”), are also considered examples of ilhad.