High crimes and misdemeanors is a phrase from the United States Constitution, Article II, Section 4: "The President, Vice President and all civil officers of the United States, shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanors."
"High" in the legal parlance of the 18th century means "against the State". A high crime is one which seeks the overthrow of the country, which gives aid or comfort to its enemies, or which injures the country to the profit of an individual or group. In democracies and similar societies it also includes crimes which attempt to alter the outcome of elections.
The first impeachment conviction by the U.S. Senate was in 1804 of District Judge John Pickering for the high crime and misdemeanor of chronic intoxication. Federal judges have been impeached and removed from office for tax evasion, conspiracy to solicit a bribe, and making false statements to a grand jury.
In the impeachment of Bill Clinton in the late 1990's, the exact meaning of the term "High crimes and misdemeanors" became the subject of debate. A particular subject of debate is exactly what rises to the level of "High crimes and misdemeanors". Those in favor of Clinton's impeachment felt that the acts he was being impeached for certainly rose to that level, while those opposed felt that his acts, while still wrong, did not reach that level.