For this essay, you are asked to define "science." The question you are to answer is simple:
"What is science?"
Your audience are nonscientists: scientifically interested but naïve adults who do not understand why so-called "scientific claims" they may see on TV infomercials are not science. Use the William Speed Weed essay as a guide; the people who believe weight-loss pills will help them lose weight without exercising are your target audience.
Do not talk down to your audience. Instead, assume they are misinformed (not merely uninformed). Explain what science is, and how it differs from non-science.
You may define by example (and counter-example), by comparison (and contrast), by process ("what does science do?"), but you should not rely on dictionary definitions. I do not want to see any essays which begin with any variations on "Webster's Dictionary defines 'science' as . . . "
You may certainly use dictionaries and encyclopedias as initial sources, but the definition essay has to provide concrete examples of what science actually is and what it does.