Generally, an essay means without any limitation. Something that is free from all the limitations is an essay- sky is the only limit
Nevertheless, there is another meaning of essay. That is the opposite of the above that is, perfectly bounded- in a well built cage.
I am assuming that you are confused after reading both the meanings since both of them contradict each other. On one hand it is a free bird and on the other, it is a parrot in the cage. The problem in front of you is which of the above you should take as correct for an essay in order to start your preparation. Don't worry; I will help you solve this problem.
Both the meanings of an essay, which appear to be contradictory, somehow support each other too. The first meaning is related to the contents of the essay, whereas the second one is related to the writing style, expression and its structure. I mean to sayan essay can be written on any topic-from nuclear test, God particle and Mars orbiter to environment,poverty, inflation and success in life etc. This can even include topics like agriculture, entertainment, sports, science and even quantum physics. There is no limit to the essay's topic. This way it is free and without boundaries.
However, the freedom of topic doesn't mean that 'anything' can be written. You are given the liberty to write anything related to the topic. But, whatever is written should be somehow related to the topic. It is like a cow which is tied with a nail in a ground with a rope which is 15 meters long. It is free in the sense that it has enough space to roam around. Nonetheless, it is confined within the distance of 15 meters.
The second boundation is that of its composition. An essay doesn't mean anything can be written anytime and anywhere. It has its own structure in which we have to say what we want to say. Hence, we can say an essay has two limitations- one is that of opinions and thoughts and the second is that of its style.