A serif is a delicate little point (aka flare or terminating flourish) at the end of a letter stroke. In the image above, we've circled the serifs around the 'A' and 'a' on the left.

The watch’s notable visible feature is its sophisticated and spare sans-serif typeface, which features numerals distinguished by subtle line breaks. Call it ‘‘Font, Interrupted.’’ ...

So long, Calibri: Microsoft has settled on a new font for its Office apps Aptos is a sans-serif font that also comes in monospaced and serifed varieties.