The format for a number written in base-16 instead of base-10 (decimal). Commonly used in computing or electronic fields, since each digit in a hexadecimal number is trivially converted (by humans or machines) into 4 bits with a table. A number in base-10 is more difficult to convert into binary, requiring repeated division by 2.
When writing a hexadecimal number, for digits greater than '9' convention dictates use of the first 6 letters of the alphabet. So the digits are:
0 1 2 3 4 5 6 7 8 9 A B C D E F
The value of the popular magic number DEADBEEF when converted from hexadecimal to decimal is 3735928559.
A constant compiled into a computer program or used in a file format for the purpose of being identifiably unique. A number which could only exist in memory on purpose and rarely by accident. For example, 0 is a bad choice for a magic number, since it's the default value for memory when a computer is turned on.
Programmers use them to quickly validate a larger data structure which could crash if it guesses the type wrong, using the theory that whatever code put the magic number there probably got the rest of it right, since they would have made it store the magic number AFTER everything else checked out.
Usually it's something easy for a programmer to remember, like DEADBEEF or B00B135 or something equally assinine.
It's like a secret signal or whistle you might use with friends when trying to coordinate something sneaky.
I fed an mp3 I accidentally renamed to a avi into windows media player, but it didn't crash because it checked the magic number first.