1. Someone who challenges or discredits another person's honor, reputation, etc.
2. Anyone who challenges the practices or honesty of; to accuse; especially, to bring (a public official) before a proper tribunal or legal authority on a charge of wrong-doing.

Whenever anyone demands that a public official should be impeached, that person usually intends for the official to be immediately removed from office.

This general use of impeach as a synonym of "throw out" does not agree with the legal meaning of the word. When a public official is impeached or formally accused of malfeasance, it is just the beginning of what can be a lengthy process that may or may not lead to the official's removal from office.

Legal usage refers to an official who is impeached (accused), tried, and then convicted or acquitted.