A number of fonts (Gotham, Vera, Nexa) have an uppercase M where the middle V shape doesn't touch the baseline. In many (most?) other fonts, that shape does touch the baseline. Is there a formal or informal term for this difference (à la "two story a")?
You could call the V shape in a letter M the "vertex".
On Identifont they use this terminology, and call the V shape in the M a "centre vertex".
They describe it like this
The upward pointing corners of the M are sometimes called an "apex", so technically, the M only has one vertex, so there's no need to call it a "centre vertex". You could just say "the vertex is above the baseline" to describe it.