anything indispensable; "food and shelter are necessities of life"; "the essentials of the good life"; "allow farmers to buy their requirements under favorable conditions"; "a place where the requisites of water fuel and fodder can be obtained"
This is a common word used in a peculiar way by medical insurance companies and which results in great confusion. When an insurance company uses the word “necessary” they indicate that they are willing to pay. If they say “not necessary” they mean that they will not pay. It has nothing to do with a patient's need for a treatment. For example, an insurance denial form may use the abbreviation, “MED PROC NOT NEC” to indicate that they have decided that the procedure is “not necessary” for the patient, and that they do not wish to pay for any charges.
The little blue porta-potties/outhouses/chemical toilets found at reenactments. "The necessary" was actually a period term for outhouse. Also called Comfort Station, Hooters, Irish Shanty, Officer's Quarters, and Our Lady of the Blue Waters.