In "Macbeth" how does Shakespeare define what it means to be human?
In Macbeth, Shakespeare tells us that the tragic hero surrenders everything that it means to be human in order to pursue the possession and ownership of power, the crown or throne of Scotland.
To be human means, you enjoy the company of friends, like Banquo, who he has murdered. You have a loving relationship with your wife, something that he loses when he commits to the pursuit of power through the use of evil. You sleep, peacefully, without nightmares. You wake refreshed and rested.
"After he has killed Duncan, his conscience begins to project voices that he thinks the entire castle can hear. “Still it cried ‘Sleep no more!’ to all the house;/ ‘Glamis hath murdered sleep, and therefore Cawdor/ Shall sleep no more; Macbeth shall sleep no more.’"
You eat and enjoy your food. Your mind is at peace, you do not worry constantly. You don't see hallucinations or hear voices. You don't feel possessed by a sense of fear and anxiety that commands that you murder. The ability to relax and enjoy the moment.
He gave up his sanity. His grip on reality.
Macbeth gave up all that which made human life enjoyable.
pmiranda2857 has already answered your question excellently: though it is the crown of Scotland, not England which Macbeth longs for and eventually gains.
And, interestingly, Macbet himself summarises the things he's missed out on in a speech in Act 5, Scene 3:
My way of life
Is fall'n into the sear, the yellow leaf,
And that which should accompany old age,
As honor, love, obedience, troops of friends,
I must not look to have; but, in their stead,
Curses, not loud but deep, mouth-honor, breath,
Which the poor heart would fain deny and dare not.
Hope it helps!