Only a fraction of a body seats are up for election while others are not until the terms of the next set of members are to expire.
The legislative bodies of the United States are the Senate (which serves six-year terms) and House of Representatives (two year-terms).
Regarding elections to the United States Congress, the point of reference is the president's term.
This is especially the case in the Philippine Senate, when there are elections every three years to fill half of the 24 seats for a six-year term.
In European countries where parliamentary systems of government are prevalent, midterms do not always exist in the strictest sense.