A length of (leftover) yarn coiled into a self-contained figure-8. In a center-pull bobbin, you pull the working yarn from the center.
The form which a ball of yarn has when it is wound to enable you to pull the working yarn from the center.
The shape that yarn is twisted into to make it compact for dyeing and displaying. Hanks must be wound into balls before you can knit with them. Most people incorrectly use the word “skein” for yarn in this shape. I recommend you do the same (unless you’re writing a blog post or something where people will call you out).
A ball of yarn. The term “skein” is often used erroneously to mean “hank,” while “ball” is a more common word for a skein.