Okay, so I know it's not a highly unusual word, but it IS the word of the day at dictionary.com.
complement \KOM-pluh-muhnt\, noun:
1. Something that fills up or completes.
2. The quantity or number required to make up a whole or to make something complete.
3. One of two parts that complete a whole or mutually complete each other; a counterpart.
1. To supply what is lacking; to serve as a complement to; to supplement.
I've got some good ideas for how to represent this word in Etsy items, but I thought it would be fun this time to wait a day until I posted my finds. What item(s) do you think best represent this word? If you've got something to add to the list, email me or post a comment. Then, I can post all of them together.