The bystander effect is a social psychological phenomenon that refers to cases where individuals do not offer any means of help in an emergency situation to the victim when other people are present. The probability of help has in the past been thought to be inversely related to the number of bystanders; in other words, the greater the number of bystanders, the less likely it is that any one of them will help. The mere presence of other bystanders greatly decreases intervention. This happens because as the number of bystanders increases, any given bystander is less likely to interpret the incident as a problem, and less likely to assume responsibility for taking action.
You likely witness the bystander effect quite frequently. Most situations are not serious, such as someone dropping their books in the middle of a crowded Coffman while dozens of people walk by doing nothing. However, sometimes the bystander effect can cause more serious problems. In every CPR course I've ever taken, the instructor stressed how important it is to designate a specific person in the crowd to call 911. This is because due to the bystander effect, in large crowds, no one feels the need to take responsibility because they rely on others to do so.