A zwitterion is a molecule with regions of a formal positive and negative charge. This is for example the usual state of polypeptides at neutral pH in solution, with -NH3+and -COOH- terminal groups.

Definition:
A zwitterion is a dipolar ion, i.e. an ion that has both positive and negative regions of charge; amino acids for example, occur as zwitterions in neutral solution, and the pH value at which the zwitterion state is at a maximum is the isoelectirc point.
(Dorland's Medical Dictionary (hardback))

Hence. in a zwitterion a "charge of opposite sign" is a charge which is both positive and negative.

In chemistry, a formal charge (FC) is the charge assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative electronegativity.
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