essence - (n.): the most basic and important quality of something; the most significant trait of a person or thing

• Synonyms: meaning, heart, core, quality
• Origin: 1398, from L. essentia meaning "being," which comes from the Greek professor Aristotle, who used the phrase to ti esti, meaning “what it is”
• Example: Just looking at this candle, I can almost taste and smell the essence of the citrus fruits from which it was made.

Just walking into our room, a guest could almost feel the essence of the students who work and learn here.  

How would you describe the essence of our class?