What is impedance?

Some articles on impedance:

Composite Image Filter - Filter Section Types - Effect of End Terminations
... Further into the passband the impedance match progressively improves, thus limiting the error ... So while stopband impedance mismatch may be severe, it has only limited effect on the filter response ... k-type low-pass T-filter (two half-sections) response when correctly terminated in image impedance Practical k-type low-pass T-filter (two half-sections) response when terminated with fixed resistors ...