Begin your essay with a thesis. Divide your essay into three parts: an introduction, a body and a conclusion. The thesis is the focal point of the entire essay, so it has to be a part of the introduction. In the definition essay, the thesis is always the definition of the term you are explaining. "Friends is a popular TV series from the 1990s" is an example of what your thesis for this essay can be.
Use the body of your essay to introduce facts that support your thesis. Since you are defining the TV show "Friends," you should introduce the cast and the characters. Explain which actor plays which character, and give a short description of each character. This exposition will underline the fact that you are writing about an acted and scripted TV series.
Explain where the show was filmed. Write about how the show is based in New York, describe the two apartments and explain to the reader that the show was filmed in front of a live studio audience.
Write about the popularity of the show. Over 20 milion people watched every season finale, which makes "Friends" one of the most successful TV series of its time. The shows immense popularity gave the actors so much leverage they were able to negotiate $1 million fees per episode with NBC.
Conclude with an overview. The conclusion of the essay serves as a quick summary of everything you wrote. Restate your thesis and all your points in the same order in which they appear in the paper. Keep the recap short. Don't write more than a sentence on each of your points. Finish the essay on an interesting note. For example, you can briefly describe the show's influence on fashion trends of the time or on the actors' careers after the series ended.