An intermediate-range ballistic missile (IRBM) is a ballistic missile with a range of 3,000-5,500 km (1,865-3,420 miles), between a medium-range ballistic missile and an intercontinental ballistic missile. Classifying ballistic missiles by range is done mostly for convenience, in principle there is very little difference between a low-performance ICBM and a high-performance IRBM. The range definition used here is used within the U.S. Missile Defense Agency. Some other sources include an additional category, the long-range ballistic missile (LRBM), to describe missiles with a range between IRBMs and true ICBMs. The more modern term theater ballistic missile encompasses IRBMs, MRBMs and SRBMs, including any ballistic missile with a range under 3,500 km (2,175 mi).