A looking glass is simply a mirror, though the term sounds more old-fashioned and poetic than the everyday word we use now. When Lewis Carroll wrote Through the Looking-Glass in 1871, he was describing Alice stepping through a mirror into a backward world where everything was reversed.

People once called mirrors “looking glasses” because early mirrors were made by coating glass with reflective metal on the back. You would look into the glass to see your reflection. Before glass mirrors became common in the 1600s, people used polished metal or other reflective surfaces to see themselves, but these didn't work nearly as well.

Today, we rarely say “looking glass” in normal conversation. We just say mirror. But you'll still encounter the term in older books, poems, and historical contexts. It can add a touch of wonder or mystery to writing, which is probably why Carroll chose it for his story about Alice's strange adventure on the other side of the looking glass.