Remote sensing involves the use of extracting information about a geographic area through the use of aerial or satellite imagery. Laurence Smith defines remote sensing:
Remote-sensing technologies come in two flavors: Passive remote sensing relies on naturally reflected or emitted energy of the imaged surface (think of taking a photograph with a camera under sunlit conditions). Most remote sensing instruments fall into this category, obtaining pictures of visible, near-infrared and thermal infrared energy. Active remote sensing means that the sensor provides its own illumination and measures what comes back (think of a camera with a flash). Remote sensing technologies that use active remote sensing include lidar (laser) and radar.
While the origins of remote sensing can be traced to World War II’s use of radar, sonar, and thermal detection technologies , the use of the term ‘remote sensing’ wasn’t coined until the late 1950s. Evelyn Pruitt, a geographer with the U.S. Office of Naval Research, was the first to coin the term ‘remote sensing’. It was a need to define the emerging imaging capabilities of multispectral cameras, infrared films, and nonphotographic scanners that prompted Puritt to come up with a name for this new field of study. The term was promoted in an early 1960s white paper prepared by the staff of the Geography branch at the Office of Naval Research. 
References Moore, G. K. (1979). What is a picture worth? A history of remote sensing/Quelle est la valeur d’une image? Un tour d’horizon de télédétection. Hydrological Sciences Bulletin, 24(4), 477-485.
 Fussell, J., Rundquist, D., & Harrington, J. A. (1986). On defining remote sensing. Photogrammetric Engineering and Remote Sensing, 52(9), 1507-1511.