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An interstellar object is an astronomical object (that is not, itself a star) that is not gravitationally bound to the Sun. The term interstellar interloper more specifically refers to such an object that is passing through the solar system. Such interlopers are identified by their speed, position, and trajectory: their speed for their position is too fast for an orbit of the Sun, i.e., more than the Sun's escape velocity at that location, and their path is hyperbolic rather than elliptical. The first such body, 1I/'Oumuamua, was identified in 2017, and as of 2026, two more have been identified, 2I/Borisov (2019) and 3I/ATLAS (2025). None of the three have had a "marginal" speed: each is traveling so fast that there is no chance that it is in a solar orbit, and is following a path that is substantially different than the elliptical orbit of a solar system body: the three had orbital eccentricities of 1.2, 3.35, and 6.1.
It is presumed that such objects regularly pass through the solar system, these being discovered recently as searches for solar system objects and general monitoring of the celestial sphere has become more comprehensive. It is presumed more will be found, likely many more as the Rubin Observatory reaches normal operation.
With the discovery of Oumuamua, a means of designating such objects was developed: like minor planets, they will incorporate a number (based upon discovery order if possible, thus the "1"), with the number followed by "I" for interstellar. The three interstellar objects were spotted some time before their speed and position revealed to be an interstellar object, and they were given the normal designators for solar system objects, after which new designations were given to indicate they are interstellar, once that was established. (Such redesignation is common between minor planets and comets: it often is not initially clear whether the object is comet-like.)
Interstellar objects that are not interlopers into the solar system include free-floating planets; the "number I" designation form is not used for them, that designation being specifically for interstellar interlopers.