1. Important in order to achieve a specific result or desired by authority or convention: It is necessary that we let the local authorities know that the sewer cover on the street is missing.
2. Inevitable according to what has happened previously: The authorities will no doubt find it necessary to replace the sewer cover as soon as possible.
3. Relating to being essential, indispensable, or a definite requirement: The mechanics at the repair station had the necessary tools to fix the cars that their customers brought in for repairs.
4. Etymology: from Latin necessarius, from necesse, "unavoidable, indispensable"; originally, "no backing away" from ne-, "not" + cedere, "to withdraw, to go away, to yield".