Defamation occurs when one person publishes a false statement that tends to harm the reputation of another person. Written defamation is called libel. Spoken defamation is called slander.
If your employer says something that is false about you to someone else such as a perspective employer then you may, under some situations, have a claim for defamation. Your employer does have some defenses depending on the circumstances that the statements were made. Roger Davie can evaluate your defamation / libel / slander claim.