esp. is the written short form of especially. It means “most of all” or “more in this case than in others”: the thing is true in general, and even truer here. You meet it in notes, dictionaries, lists, and little asides in brackets, where space is tight. Keep the full stop. When you read it aloud, you almost always say the full word especially, not the letters; though actually someone spelling it out might say E-S-P. It is a writing shortcut, not a different word, and it is a little casual in a careful school essay — teachers often prefer especially written in full there.

尤其是、特別是（書面縮寫）