in essence

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.

The adjective has one meaning:

Meaning #1: being the essence or essential element of a thing
  Synonyms: substantial, substantive

The adverb in essence has 2 meanings:

Meaning #1: at bottom or by one's (or its) very nature
  Synonyms: basically, fundamentally, essentially, au fond

Meaning #2: with regard to fundamentals although not concerning details
  Synonyms: in principle, in theory