urchin vs street urchin

urchin and street urchin both are nouns.

Word	Noun	Adjective	Verb	Adverb
urchin	Yes	No	No	No
street urchin	Yes	No	No	No

As nouns, street urchin is a hyponym of urchin; that is, street urchin is a word with a more specific, narrower meaning than urchin:

urchin: poor and often mischievous city child
street urchin: a child who spends most of his time in the streets especially in slum areas

Other hyponyms of urchin include ragamuffin, tatterdemalion, guttersnipe.

urchin (noun)	street urchin (noun)
poor and often mischievous city child	a child who spends most of his time in the streets especially in slum areas