What does it mean for an argument to be valid?

An argument is valid if and only if the following is impossible: all of the premises are true and the conclusion is false. E.g. the argument below is valid:(1) All men are mortal(2) Socrates is a man(3) So, Socrates is mortalThe argument is valid because it cannot be the case that (1) and (2) are both true while (3) is false. The following argument, by contrast, is invalid:(1) Some men have beards(2) Socrates is a man(3) So, Socrates has a beardThis argument is invalid because it could be the case that (1) and (2) are both true while (3) is false. After all, it could be the case that Socrates is one of those men who don't have a beard.