1.
absolutely essential
2.
unavoidably determined by prior circumstances
the necessary consequences of one's actions
'necessary' - used as a noun
3.
anything indispensable
The following are synonyms for 'necessary'. Click on any of them to get both Bibliodata's full list of common definitions and a further set of synonyms.
The Pentagon, in Arlington, Virginia, has twice as many bathrooms as is necessary. When it was built in the 1940s, the state of Virginia still had segregation laws requiring separate toilet facilities for blacks and whites.