Wikipedia
A is a drinking party phenomenon particular to Japanese culture. Nomikai are a part of the culture of most places of employment, from schools to nightclubs. They are most often held in restaurants or izakaya, usually with everyone seated at one large table or occupying a separated section of the venue.
Employees are usually expected to participate to some extent in various nomikai, as it is considered a social aspect of work, although it is not expressly required. Such parties focus on the bond between coworkers as a group, and are not considered private or somehow non-work related (see Japanese values). Attendance does not necessarily imply that a person will drink any alcohol, however, and attendees generally pay a set amount for the food, drink, and venue regardless of consumption. Any leftover money may be put toward organizing the next nomikai.
The student version of the nomikai is called a konpa and contains some distinguishing characteristics.