a great amount: a large number of things, or a large degree of something
This is the noun for sheer plenty. It does not name what is plentiful, it names the plenty itself, and so it leans on whatever word follows it: an abundance of grain, a multitude of people, the greatness of God's mercy. The same word covers both things you can count and things you cannot, because in Hebrew the line between many and much is not drawn the way English draws it.
When the thing in view is countable, it means a great number: troops too many to count, a crowd, a swarm of locusts, a flood of words. When the thing is not countable, it shifts to a great degree: the depth of someone's strength, the fullness of wealth, the vastness of God's faithful love. In both cases the point is the same, that there is a lot of it.
The word turns up most in two everyday phrases. After a small connecting word it reads as in great quantity, said of silver as common as stones or cattle beyond counting. After another small word it reads as because of the greatness of, as in because of the greatness of your mercy. Both are simply this noun put to work as a measure.
Because it borrows its meaning from its companion word, English translators reach for many different renderings, multitude, abundance, greatness, plenty, length, depending on what is being measured. The underlying Hebrew word is one and the same.