bulb vs round shape

bulb and round shape both are nouns.

Word	Noun	Adjective	Verb	Adverb
bulb	Yes	No	No	No
round shape	Yes	No	No	No

As nouns, round shape is a hypernym of bulb; that is, round shape is a word with a broader meaning than bulb:

bulb (noun)	round shape (noun)
electric lamp consisting of a transparent or translucent glass housing containing a wire filament (usually tungsten) that emits light when heated by electricity	a shape that is curved and without sharp angles
a rounded part of a cylindrical instrument (usually at one end)
a rounded dilation or expansion in a canal or vessel or organ
lower or hindmost part of the brain; continuous with spinal cord; (`bulb' is an old term for medulla oblongata)
a modified bud consisting of a thickened globular underground stem serving as a reproductive structure
anything with a round shape resembling a teardrop