Not a word, but used in text, usually to capture an in-text sidenote, or an explanation. Tip: look at the title and you'll see.

In a darkened room, it takes the stage, A sprinkle of light, like a leprechaun's rage. Casting scenes so bright, In the midst of the night, It brings tales to life on a magical page.

One of these a day, Will keep the doctor away

Some store it for winter: both people and critters. It holds machinery together, but it can be eaten.

Spun around by gentle hand, A sphere of lands and seas expand. Spin or rest on an iron stand, A world of wonders at your command.