The European Space Agency. (2022, March). CubeSats orbiting Earth. Retrieved from https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2016/04/cubesats_orbiting_earth/15950521-1-eng-GB/CubeSats_orbiting_Earth_pillars.jpg
Short definition:
CubeSat (Cube Satellite) is a spacecraft that is meant to be a planet's artificial satellite and is designed as a cubic case. The purpose of such case is to carry its payload.
Detailed Definition
CubeSat is a nanosatellite. It is built as a cube with standard dimension, length of each side is a multiple of 10cm or 1U. They can contain arbitrary payload, usually scientific equipment such as small cameras. Their usual weight is about 1.3 kilograms, what makes them relatively cheap to launch and hence widely accessible. Their accessibility allows smaller players in space industry such as companies and even universities to deploy their space exploration tools into space.
Etymology
Cube SatelliteCube – from Latin cubusSatellite – from Latin satellitem– an attendant