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A Composite number which is the Product of two Primes (possibly equal). They correspond to the 2-Almost Primes. The first few are 4, 6, 9, 10, 14, 15, 21, 22, ... (Sloane's A001358).
See also Almost Prime, Chen's Theorem, Composite Number, Prime Number
References
Sloane, N. J. A. Sequence
A001358/M3274
in ``An On-Line Version of the Encyclopedia of Integer Sequences.''
http://www.research.att.com/~njas/sequences/eisonline.html and Sloane, N. J. A. and Plouffe, S.
The Encyclopedia of Integer Sequences. San Diego: Academic Press, 1995.