Definition from Wiktionary, the free dictionary
See also a1
Etymology 1 
See the Wikipedia article ISO 216, the international standard on paper sizes.
- A standard paper size, defined by ISO 216 to have dimensions: 841 x 594 mm (840.90 x 594.60 mm)
- a sheet of A1 paper
See also 
Etymology 2 
From various rating systems, as that of Lloyd's of London or the condition of insured ships. First attested in 1837.
Alternative forms 
A1 (not comparable)
- (informal) In good health.
- (informal) In excellent or top condition.
- (of a ship) First-class. (Compare A 2 and A 3, which are inferior grades.)
Etymology 3 
Proper noun