Why That They Called Undefined Terms?

An undefined term is one that has no definition, thus no meaning.

Examples
1/0 is undefined (there is no number x such that 0*x = 1)
√-1 is undefined for real numbers (there is no real number x such that x² = -1)
Log[-1] is undefined for real numbers (there is no real number x such that 10^x = -1