Wiktionary
b-flat major
n. 1 (context music English) The major key with B-flat as its tonic, consisting of the pitches B♭, C, D, E♭, F, G, and A. 2 (context music English) The major chord with a root of B-flat.
Wikipedia
B-flat major
The '''B (B-flat) major scale ''' consists of the pitches B, C, D, E, F, G, and A. Its key signature has two flats.
Its relative minor is G minor, and its parallel minor is B minor.
Many transposing instruments are pitched in B-flat major, including the clarinet, trumpet, tenor saxophone, and soprano saxophone. As a result, B-flat major is a popular key for concert band compositions.