Final answer:
The term for failing to help someone because many witnesses are present is known as the bystander effect, which involves diffusion of responsibility.
Step-by-step explanation:
When you fail to help someone in a crowd because there are many other witnesses, this is referred to as the bystander effect, which is closely associated with diffusion of responsibility. This phenomenon occurs as individuals feel less compelled to take action as they assume that someone else will step forward to help. Hence, the responsibility is spread across all the witnesses. Researchers have found that one is more likely to help if they are the only one around or if they know the victim, but less likely when surrounded by others who could also potentially intervene.