Text describing an image for those who cannot see it. The image's meaning made available by another route.

A likeness that stands in for the thing itself. An image trusted to carry meaning without explaining it.

A likeness of a particular person. Also taller than wide — the orientation named for the pictures it suits.

That which turns a description into something perceivable. It adds no meaning; it makes existing meaning visible.

A format describing images as shapes rather than as dots, so they scale without loss. A named format rather than a concept.