The poet reminds men of their uniqueness and it is not necessary to possess the ultimate definition of this uniqueness. Even to speculate is a gain.

-Cousins, Norman
  In the Saturday Review,15  Apr.

   'Girl number twenty unable to define a horse!'said Mr Gradgrind† 'Girl number twenty possessed of no facts, in reference to one of the commonest of animals!'† 'Bitzer'said Thomas Gradgrind.'Your definition of a horse.' 'Quadruped.Graminivorous. Forty teeth, namely twenty-four grinders, four eye-teeth, and twelve incisive. Sheds coat in the spring; in marshy countries, sheds hoofs, too. Hoofs hard, but requiring to be shod with iron. Age known by marks in mouth.' Thus (and much more) Bitzer. 'Now girl number twenty,'said Mr Gradgrind.'You know what a horse is.'

-Dickens, CharlesJohn Huffam
  Hard Times, bk.1, ch.2.

He did the thing that no one else would even dare to think of doing. And that to me's as good a definition of a 'hero'as you'll get. Even when the thing that's done is something of which you disapprove.

-50 Cent originally  CurtisJackson
   Juliet D'Orsey. The Wars, pt.1, section 3.

My definition of utter waste is a coachload of lawyers going over a cliff, with three empty seats.

-Hunt, Lamar
Quoted in Colin  Jarman The Guinness Dictionary of Sports Quotations (1990).

The definition of happiness of the Greeks†is full use of your powers along lines of excellence. I find, therefore, the Presidency provides some happiness.

-Kennedy,John F(itzgerald)
  News conference, 31 Oct.

The definition of a specialist as one who'knows more and more about less and less' isgood and true.

-Mayo, Charles Horace
  In Modern Hospital, Sep.

'Good', then, if we mean by it that quality which we assert to belong to a thing, when we say that the thing is good, is incapable of any definition, in the most important sense of the word.

-Moore, G(eorge) E(dward)
  Principia Ethica, ch.1.

No man, not even a doctor, ever gives any other definition of what a nurse should be thanthisö'devoted and obedient'. This definition would do just as well for a porter. It might even do for a horse.It would not do for a policeman.

-Nightingale, Florence
  Notes on Nursing.

DefinitionöScience is systematized positive knowledge, what has been taken as such in different ages and in different places. TheoremöThe acquisition and systematization of positive knowledge are the only human activities which are truly cumulative and progressive.CorollaryöThe history of science is the only history which can illustrate the progress of mankind. In fact, progress has no definite and unquestionable meaning in other fields than the field of science.

-Sarton, George A
  The Study of the History of Science.

My definition of a free society is a society in which it is safe to be unpopular.

-Stevenson, Adlai E(wing)
  Speech, Detroit, Oct.

If you want a definition of poetry, say: 'Poetry is what makes me laugh or cry or yawn, what makes my toenails twinkle, what makes me want to do this or that or nothing'and let it go at that.

-Thomas, Dylan Marlais
  Letter to a student.