An Armstrong number, also known as a 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 itself. These numbers can be found in various bases, but the most common base is decimal (base 10). Some other examples of Armstrong numbers include 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 370, 371, and 407. They are often used in recreational mathematics and number theory.