'Regarding third-party cookies, who is a "second party"?

I am new to web dev. I don't think web dev is difficult. As long as you find a good explanation, things are crystal clear.

Unfortunately most tutorials you came across are not doing a good job.

First-party cookie is very easy to understand.

I am having trouble understanding third-party cookies.

Why is it called third-party?

Who is the second party, which is being skipped here?

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