A midterm election refers to a type of election where the people can elect their representatives in the middle of the term of the executive or of another set of members. This is usually used to describe elections to a governmental body (upper house, legislature, etc.) that are staggered so that the number of offices of that body would not be up for election at the same time. Only a fraction of a body's seats are up for election while others are not until the terms of the next set of members are to expire. The legislators may have the same or longer fixed term of office than the executive, which facilitates an election mid-term of the tenure of the higher office.
In the United States, the president is elected every four years in indirect (electoral college) presidential elections. The legislative bodies of the US are the Senate (who serve six year terms) and House of Representatives (2 year terms). The senate has 1/3 of its members up for election every two years while the House has all its membership up for election every two years. Regarding elections to the United States Congress, the point of reference is the president's term. There are three classes of United States Senators; each election replaces one class, hence a "midterm election" appears as one-third through the term of one class and two-thirds through the other, while still midway the term of a president. While the Philippines and Liberia also conducts midterm elections, the winners of such elections take office in their respective legislative bodies that conduct such elections midway through the term of half of the other members, hence for the members who were not up for election, the incoming members take office midway through their terms.
While House of Councillors of Japan uses a staggered election, there are no fixed terms to compare with as the House of Representatives has a variable term, and the position of the emperor is hereditary.
The results of such a midterm election serve as a measuring stick to the popularity of the incumbent executive, although in the United States the ruling party has suffered election defeats for most of the time.