The phrase "essentially define" is correct and can be used in written English.
It is typically used to describe the most important or fundamental aspect of something, or to summarize the definition of something in a concise manner. Example: The purpose of this essay is to essentially define the concept of democracy and its principles.
"It occurred to us that the items in your handbag essentially define who you are and what is important to you," Ms. Spade said.
Fadl then establishes a new set of rules for jihad, which essentially define most forms of terrorism as illegal under Islamic law and restrict the possibility of holy war to extremely rare circumstances.
Some right-wing politicians have recently called for Israel to assert legal sovereignty over the territory, a unilateral step that would essentially define the contours of a future Palestinian state — and that is anathema not only to the Palestinian leadership but also to the United States and Europe.
In general, RA problems in wireless communication essentially define how to optimise the limited resources in the communication network.
These two benefits essentially define the two categories of fMRI analyses, task-evoked fMRI and resting-state fMRI.
A parametrized model in addition to the control and state-space variables depends on time-independent design parameters, which essentially define a family of models.
Until now Nintendo has essentially defined this market.
From Mafia Wars to FarmVille, Zynga has essentially defined the latest generation of games on Facebook.
"Sir William Osler is the man who essentially defined medical science as we're still doing it 150 years later.
Instead, it has become something of a roadblock in the folk-based singer-songwriter genre that Dylan essentially defined.
"The company essentially defined what the modern MP3 player, smartphone and tablet computer looks like and what their functions are.
