Specular Highlight
A mirror-like reflection of the light source itself on a glossy surface, rather than light diffusely scattered by it.
In depth
Speculars carry no detail of their own — they are an image of the source — so their size, shape and position are controlled entirely by the modifier and its placement. On skin they read as sheen, on metal and glass they define the form completely, which is why product photographers shape large soft sources specifically to produce the right reflection.
Examples in practice
A long strip softbox reflected down the side of a wine bottle draws a clean vertical highlight that describes its curve.