An Armstrong number (or narcissistic number) is a special type of number that is equal to the sum of its own digits raised to the power of the number of digits. For example, the number 153 has three digits, and when you calculate 1^3 + 5^3 + 3^3, the result is 153, making it an Armstrong number. These numbers can be found in various bases, but the most common base is decimal (base 10). Other examples include 0, 1, 370, and 9474. Armstrong numbers are often used in programming and mathematics to illustrate concepts of number theory and digit manipulation.