nb

(noun) a soft grey ductile metallic element used in alloys; occurs in niobite; formerly called columbium. Synonyms: niobium, Nb, atomic number 41.

(noun) a Latin phrase (or its abbreviation) used to indicate that special attention should be paid to something. Synonyms: nota bene, NB, N.B.. Example: "The margins of his book were generously supplied with pencilled nbs."