To the phenomenon in which the presence of other people makes it less likely that any individual will someone in distress because the obligation to intervene is shared among all onlookers.

The answer is "Diffusion of responsibility".

Diffusion of responsibility is a sociopsychological phenomenon whereby a man is less inclined to make obligation regarding move or inaction when others are available. Thought about a type of attribution, the individual accept that others either are in charge of making a move or have officially done as such. Diffusion of responsibility happens when individuals who need to settle on a choice sit tight for another person to act. The more individuals included, the more probable it is that every individual will do nothing, trusting another person from the gathering will likely react.