From [link|http://www.law.cornell.edu/background/impeach/|Legal Information Institute's Backgrounder on Impeachment]:
The House of Representatives considers and debates the articles of impeachment. A majority vote of the entire House is required to pass each article. Once an article is approved, the President is, technically speaking, "impeached" -- that is subject to trial in the Senate.
So "impeached" roughly means "indicted". It just means that there will be a trial in the senate. If that trial yields a "not guilty" verdict, the president doesn't have to reclaim office, as he was never removed from office.

Although, from the same link as above:
Technically, impeachment is the Senate's quasi-criminal proceeding instituted to remove a public officer, not the actual act of removal. Most references to impeachment, however, encompass the entire process, beginning with the House's impeachment inquiry.