narrow margin is a noun meaning "a small margin". It carries one recorded sense.
a small margin
“the president was not humbled by his narrow margin of victory”
“the landslide he had in the electoral college obscured the narrowness of a victory based on just 43% of the popular vote”
Definitions, synonyms and semantic relations for narrow margin come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.