1
general::
Basically, by nature, as in He is in essence a very private person or In essence, they were asking the wrong question. This term employs essence in the sense of "intrinsic nature," a usage dating from the mid-1600s.
American Heritage Idioms
2
general::
Phrase(s): in essence
basically; essentially. • I have lots of detailed advice for you, but in essence, I want you to do the best you can. • In essence, lightning is just a giant spark of electricity.
McGrawhill's American Idioms And Phrasal Verbs