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A meromorphic function is a function that is holomorphic everywhere on the complex plane except at points in a set of isolated poless, which are certain well-behaved singularities. Every meromorphic function can be expressed as the ratio between two entire functions (with the denominator not constant 0): the poles then occur at the zeroes of the denominator.
Examples of meromorphic functions are all rational functions such as f(z) = (z3 − 2z + 1)/(z5 + 3z − 1), the functions f(z) = exp(z)/z and f(z) = sin(z)/(z − 1)2 as well as the Gamma function and the Riemann zeta function. The functions f(z) = ln(z) and f(z) = exp(1/z) are not meromorphic.
In the language of Riemann surfaces, a meromorphic function is the same as a holomorphic function from the complex plane to the Riemann sphere which is not constant ∞. The poles correspond to those complex numbers which are mapped to ∞.