Willful ignorance is the state and practice of ignoring any sensory input that appears to contradict ones inner model of reality. At heart, it is almost certainly driven by confirmation bias.
It differs from the standard definition of ignorance which just means that one is unaware of something in that willfully ignorant people are fully aware of facts, resources and sources, but refuse to acknowledge them. Indeed, calling someone "ignorant" shouldnt really be a pejorative, but intentional and willful ignorance is an entirely different matter.
It is sometimes referred to as tactical stupidity.
Depending on the nature and strength of an individuals pre-existing beliefs, willful ignorance can manifest itself in different ways. The practice can entail completely disregarding established facts, evidence and/or reasonable opinions if they fail to meet ones expectations. Often excuses will be made, stating that the source is unreliable, that the experiment was flawed or the opinion is too biased. More often than not this is simple circular reasoning: I cannot agree with that source because it is untrustworthy because it disagrees with me.
In other slightly more extreme cases, willful ignorance can involve outright refusal to read, hear or study, in any way, anything that does not conform to the persons worldview.