'Seed' in the Bible

Now to Abraham were the promises spoken, and to his seed. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.

What then is the law? It was added because of transgressions, till the seed should come to whom the promise hath been made; and it was ordained through angels by the hand of a mediator.

And if ye are Christ's, then are ye Abraham's seed, heirs according to promise.