
Near-rectilinear halo orbit

A near-rectilinear halo orbit (NRHO) is a type of halo orbit that has slightly curved, so near straight sides, between close passes with an orbiting body. A use of such an orbit is currently planned in cislunar space but, as of early-2021, has not been used on any spacecraft. This planned Moon-centric orbit will serve as a staging area for future lunar missions. However, an NRHO need not be involved with the Earth-Moon system, and the orbit could be used in a variety of other contexts around other bodies in the Solar System and beyond.
NRHO orbits are one theoretical solution to the classic three-body problem in gravitational mechanics. 
Planned NRHO usage.
By 2018, NASA had begun considering use of a near-rectilinear halo orbit for a future lunar mission, 
and by 2020, an NRHO is the planned orbit for the NASA Lunar Gateway, to be orbiting Earth-Moon in circa 2024. The Gateway orbit is planned to be a highly-elliptical seven-day NRHO around the Moon, which would bring the small space station within of the lunar north pole at closest approach and as far away as over the lunar south pole.
By 2019, the company Advanced Space was building a 12-unit cubesat to fly on a Gateway precursor mission for NASA. Named CAPSTONE (Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment), the spacecraft is expected to be the first spacecraft to operate in a NRHO lunar orbit in early 2021. The mission objective is to test and verify the calculated orbital stability planned later for the Lunar Gateway space station, and the spacecraft will fly the identical orbital parameters planned later for Gateway. It will also test a navigation system that will measure spacecraft position relative to NASA's Lunar Reconnaissance Orbiter (LRO), without relying on ground stations.
By 2021, Rocket Lab was contracted by NASA to launch CAPSTONE to its first-ever lunar NRHO orbit, on Rocket Lab's small-lift launch vehicle Electron.