General:
* Neglect: This implies a lack of attention or care.
* Disregard: This emphasizes a deliberate choice to not pay attention.
* Overlooking: This suggests a passive failure to notice.
More specific:
* Inattention: This refers to a lack of focus.
* Dismissal: This implies a rejection or lack of importance.
* Contempt: This suggests a feeling of superiority and disdain.
Informal:
* Cold shoulder: This refers to deliberate avoidance.
* Snub: This implies a deliberate insult by ignoring someone.
The best noun to use depends on the specific situation and the intended meaning.