Depending on the guidelines that your school provides, the formatting of your essay will differ, but there are a few things your paper will always need. If you used other people's work in your definition essay, you will have to cite it properly. Remember to include footnotes, end notes or in-text citations. Also, you will need to have a bibliography or works cited list at the end of your paper. Your teacher or professor may also want you to annotate your works cited list by writing a couple of sentences about each of your sources and how they contributed to your essay. You will likely also have to provide a title page with the title of your essay, your name, date, class and instructor.
The introduction serves to inform the reader of the basic premises, and then to state the author's thesis , or central idea. A thesis can also be used to point out the subject of each body paragraph. When a thesis essay is applied to this format, the first paragraph typically consists of a narrative hook , followed by a sentence that introduces the general theme, then another sentence narrowing the focus of the one previous. (If the author is using this format for a text-based thesis, then a sentence quoting the text, supporting the essay-writer's claim, would typically go here, along with the name of the text and the name of the author. Example: "In the book Night , Elie Wiesel says..."). After this, the author narrows the discussion of the topic by stating or identifying a problem. Often, an organizational sentence is used here to describe the layout of the paper. Finally, the last sentence of the first paragraph of such an essay would state the thesis the author is trying to prove.  The thesis is often linked to a "road map" for the essay, which is basically an embedded outline stating precisely what the three body paragraphs will address and giving the items in the order of the presentation. Not to be confused with an organizational sentence, a thesis merely states "The book Night follows Elie Wiesel's journey from innocence to experience," while an organizational sentence directly states the structure and order of the essay. Basically, the thesis statement should be proven throughout the essay. In each of the three body paragraphs one idea (evidence/fact/etc.) that supports thesis statement is discussed. And in the conclusion everything is analyzed and summed up. 
Throughout its history humankind asked itself this question and constantly tried to answer “no”. The idea of sentient living beings who are not people is present in writings of historians, geographers and other scientists for as long as the science itself exists. Mermaids, cynocephali, various bizarre species of beastmen, antipodes: all of them were embodiments of this dream. In course of time, the limits of the known Universe expanded and it became known that no such creatures existed. The only place to seek for non-human life now is other planets. Is there any hope for finding it?