We have a running debate going on about the use of the word "impeached." By definition, impeachment is merely a formal accusation brought against a political official by a legislative group. Therefore, since Andrew Johnson was involved in an impeachment process (against him), we would say that he was "impeached." The only problem is that saying he was "impeached" implies he was found guilty — which he wasn't.
Would it then be incorrect to say he was impeached... or was he? I guess the same rule goes for "court-martial."
Thanks for emailing us at Information Please. You are absolutely correct; the word "impeachment" does in fact refer to the formal accusation of wrongdoing and does not assume guilt. For more information on the origin of the word and the judicial process check out our encyclopedia entry on impeachment, or our February 1998 Spotlight on the subject.
You are also right in assuming that "court-martial" should be used the same way. For more word usage help check out the newsgroup alt.usage.english.