Remote sensing, in simplest words, means acquiring information about an object without touching the object itself.
Conventionally, however remote sensing has come to imply that the sensor and the target are located remotely apart and the electromagnetic radiation serves as a link between the sensor and the object, the sun being the major source of energy illuminating the earth. The part of this energy is reflected, absorbed and transmitted by the surface. A sensor records the reflected energy.
Remote Sensing is performed using an instrument, often referred to as a sensor. Most of the remote sensing instruments record electromagnetic radiation (EMR) that travels at a velocity of 3x10^8 m/s from the source, directly through the vacuum of space or indirectly by reflection or reradiation to the sensor. The EMR represents an extremely efficient high-speed communication link between the sensor and the remotely located objects.
A remote sensing sensor detects changes in the amount and the properties of EMR which become valuable source for interpreting various phenomena, objects etc.
Remote sensing can then be defined as-
The technique of acquiring information about an object by a recording device (sensor) that is not in physical contact with the object by measuring portion of reflected or emitted electromagnetic radiation from the earth’s surface.