What is the definition of many?
If you have any amount of things that take up a finite space the amount of those things are also finite.
My mortgage is (finally!) paid off. That means the real estate from where I'm sitting to one meter to my left is mine. I also own the real estate beyond that, from 100 to 150 cm to my left; and in addition to those areas, I also have the real estate from 150 to 175 cm to my left, and I have the real estate from 175 to 187.5 cm to my left, and I have the real estate from 187.5 to 193.75 cm to my left, and so forth, forever. Each of those things I have takes up a finite space, and there are an infinite amount of them. Through the power of convergent series, those infinitely many finite things all add up to two meters of space.
The number of grains of sand on the earth at any given moment are finite.
That's because there's a positive lower bound on the size of a grain of sand, and because the earth is finite. If you have any amount of things that take up a finite space, and
none of them are smaller than some specified minimum size, and
they all fit into a finite space, then the amount of those things are also finite. Applying intuition you derived from your experience with finite objects to infinite sets is hazardous to your correctness.
What is the definition of infinity?
In words.
Is it something that cannot be defined using only words?
How exactly are we supposed to say anything about infinity unless it is vigorously defined?
Can it be defined as opposed to just depicted with symbols?
Certainly it can be defined using only words, but you're not going to like it. It's not an intuitive definition. People use symbols and mental imagery and hand-wavy discussion instead of rigorous verbal definitions because it helps them wrap their heads around unfamiliar non-intuitive ideas. Here we go:
A set is defined as "infinite" just in case it can be put in a "one-to-one and onto" mapping with a proper subset of itself.
Now to have any idea what that gobbledegook means, any normal person will need to see an example.
...
-3 : -3
-2 : -2
-1 : -1
0 : 0
1 : 2
2 : 3
3 : 4
...
The set on the left is all the integers. The set on the right is all the integers except for 1. The "mapping" is the rule that for any element of the set on the left specifies an element on the right, and vice versa. "One-to-one and onto" means every colon has an element from the left set on its left and an element from the right set on its right, and every element of each set shows up somewhere, and there aren't any repetitions on either side.
The point is, when you map every element of a set to an element of the same set, and you never map two elements to the same element, then, just as you'd intuitively expect, you can't ever have any elements left over that you don't map any element to,
provided it's a finite set. But it's always possible to have elements left over, such as the 1 missing from the right-side set above, whenever it's an infinite set.