An envelope is a flat paper container used to hold and protect letters or documents. When you write a letter to your grandparents or mail a birthday card to a friend, you slide it into an envelope, seal the flap, write the address on the front, add a stamp, and drop it in a mailbox.

Envelopes come in different sizes: small ones for greeting cards, standard ones for letters, and large manila envelopes for reports or photographs. Before envelopes became common in the 1800s, people folded their letters and sealed them with wax.

The word also appears in the phrase “push the envelope,” which means to test limits or try something that's never been done before. When test pilots pushed the envelope of their aircraft, they flew faster or higher than anyone had before, exploring what was possible. Today, scientists, artists, and inventors push the envelope when they experiment with bold new ideas that stretch beyond what's comfortable or familiar.