不明飞行物；幽浮(unidentified flying object (UFO))
观测者尚不能识别的各种空中物体或光学现象。第二次世界大战後，随着航空和太空事业的发展，不明飞行物受人关注。1950年代美国政府建立科学小组探测观察。报告透露，不明飞行物中有90%是天文和气象现象或者是飞机、鸟类及热气体。有时因异常气象状况。有许多尚无法解释。1960年代中期，有少数科学家认为少数不明飞行物显示有天外来客的出现。这种耸人听闻的假说在报刊上发表後，立即受到其他科学家的反对。美国空军於1968年开始进行研究，坚决摒弃天外来客说。但仍然众说纷纭，大部分美国公众和少数科学家仍然支持天外来客说。不明飞行物报告的可靠性有很大的差别。裸眼会产生错觉，雷达探测虽然在某些方面较为可靠，但不能将有形物体从流星余迹、雨和热跃变的径迹中分辨出来；况且电子干扰及如积云中潮湿气团的反射都可产生假回波。亦请参阅SETI。
English version：
unidentified flying object (UFO)
Aerial object or optical phenomenon not readily explainable to the observer. Interest in UFOs increased with developments in aeronautics and astronautics after World War II. A government panel investigating sightings in the 1950s reported that 90% coincided with astronomical or meteorological phenomena or sightings of aircraft, birds, or hot gases, sometimes under unusual weather conditions. Some remained unexplained, and in the mid-1960s a few scientists concluded that a small percentage indicated the presence of extraterrestrial visitors. This sensational hypothesis, promoted in the press, met with prompt resistance from other scientists. A U.S. Air Force UFO study begun in 1968 firmly rejected the extraterrestrial hypothesis, but a large fraction of the U.S. public, and a few scientists, still supported it. UFO reports vary widely in reliability. The unaided eye is easily fooled; radar sightings of UFOs, more reliable in some ways, may fail to distinguish physical objects from meteor trails, rain, or thermal discontinuities and are subject to radio interference. See also SETI.