Elevenses is a small snack eaten around 11 o'clock in the morning. Some things you could eat or drink at elevenses might include tea, coffee, a piece of cake, a scone, or a light sandwich.
It's called elevenses because it's typically eaten around 11 in the morning, as a snack to hold people over until lunch. Think of it as the morning counterpart to afternoon tea. The specific term is an old-fashioned British one, but the custom of eating a snack at midmorning can be found in many countries, especially in Europe. You can refer to it as elevenses even if you don't eat it at eleven, but you could also try coining your own new term: Tenses? Twelveses?