What is discreet mathematics? Urbandictionary.com tells us: "Discreet mathematics refers to the subtle study of mathematics. Discreet mathematics is characterised by furtive looks in maths textbooks disguised as pr0n and by secret maths lectures held in abandoned warehouses at midnight..."
Of course, discrete mathematics is an entirely different beast.
edit, 6:31 pm: A poster on a cryptography mailing list pointed out that cryptography is "discreet" mathematics.