What does algebra mean in math

Algebra is a question that uses any letter of the alphabet to replace a missing box, example:

4y equals 20 because 4 x 5 is 20

Algebra is solving for the unknown so

if you have 10 apples and you give 5 to your friend, how many apples would you have so that is also 10-5=x solve for x

or it could be more complicated like (x-2) / 6+4x-3=17