1. absolutely necessary, essential
vital signs/importance, sth is absolutely vital
Something 'vital' is impossible to do without, is completely necessary or essential. The 'vital organs' of the body are organs that you cannot survive without. This is an adjective and can also mean 'lively' or full of energy. "He is a vital young man." You will often hear this used in corporate contexts to talk about important processes or procedures. "Testing is a vital part of the process."