This link may be of help to you
Matrix Norm -- from Wolfram MathWorld
In general, a "norm" on a vector space is a function that assigns to each vector a number, ||v||, satifying:
1) ||u+ v|| ||u||+ ||v||.
2) ||v||= 0 if and only if v is the 0 vector.
3) ||av||= |a|||v|| for any scalar (number) a, and vector v.
The formula Captain Black gives defines the "p" norm, over , for any positive number p (p is usually taken to be an integer but that is not necessary)- though I believe it should include an absolute value:
Of course, the p-norm for p= 2 is the usual "Euclidean norm" .
If p= 1 we get the "one-norm" .
Sometimes, although it doesn't fit the formula above, we define the "0-norm" to be - that is, take the absolute value of all components, then select the largest to be the norm.