Asset
A single visual, typographic, or multimedia element used in the construction of a design project.
In practice
An asset is any single reusable piece that goes into a design: a photo, an icon, a font file, a short sound, a logo in several versions. Treating these as named, versioned assets rather than loose files means a team can find the correct one quickly and update it in one place. For an app, assets might include the launch icon at every required size, the empty-inbox illustration and the brand typeface.
Often confused with Asset Library
An asset is one element; an asset library is the organized collection where many assets are stored, named and shared.
Try this
List every asset in a recent project folder and mark the ones you could not identify by file name alone.