sandglass vs egg timer

sandglass and egg timer both are nouns.

Word	Noun	Adjective	Verb	Adverb
sandglass	Yes	No	No	No
egg timer	Yes	No	No	No

As nouns, egg timer is a hyponym of sandglass; that is, egg timer is a word with a more specific, narrower meaning than sandglass:

sandglass: timepiece in which the passage of time is indicated by the flow of sand from one transparent container to another through a narrow passage
egg timer: a sandglass that runs for three minutes; used to time the boiling of eggs

Other hyponyms of sandglass include hourglass.

sandglass (noun)	egg timer (noun)
timepiece in which the passage of time is indicated by the flow of sand from one transparent container to another through a narrow passage	a sandglass that runs for three minutes; used to time the boiling of eggs