Full Idea
If there be any meaning which confessedly belongs to the term necessity, it is unconditionalness. That which is necessary, that which must be, means that which will be whatever supposition we make with regard to other things.
Gist of Idea
Necessity can only mean what must be, without conditions of any kind
Source
John Stuart Mill (System of Logic [1843], p.339 [1974 ed]), quoted by R.D. Ingthorsson - A Powerful Particulars View of Causation 5.3
A Reaction
'It is necessary to leave now, if you want to catch the train' is a genuine type of necessity. Mill's type is probably Absolute necessity, to which nothing could make any difference. Or Metaphysical necessity, determined by all things.
Book Reference
'Oxford Handbook of Philosophy of Time', ed/tr. Callender,Craig [OUP 2013], p.89