The smallest composite Mersenne number with a prime exponent
(211 - 1 = 23 x 89).
The smallest strong base-2 pseudoprime. [Terr]
The 16th century French philosopher, Charles de Bouvelles, perhaps due to difficulties in division, cited falsely 2047 as prime.
The smallest Woodall number (2047 = 8*2^8-1) with a Woodall prime as its smallest factor.
The smallest Mersenne semiprime. [Gupta]