Impeachment
Impeachment is the first of two stages in a specific process for a legislative body to consider whether or not to forcibly remove a government official from office. The impeachment itself brings the charges against the government official. The second stage is conviction, which will determine if the official shall then be removed from office.
Article One of the United States Constitution
Clause 6: Trial of Impeachments
The Senate shall have the sole Power to try all Impeachments. When sitting for that Purpose, they shall be on Oath or Affirmation. When the President of the United States is tried, the Chief Justice shall preside: And no Person shall be convicted without the Concurrence of two thirds of the Members present.
A two-thirds supermajority of those Senators "present" is required to convict, although given the obvious importance of impeachment proceedings, there are generally few absent members. In addition, requiring a two-thirds majority of those members "present" has the net effect of making a present member's decision not to cast a vote either way the same as a vote against conviction