1. An instrument for recording vibrations of the Earth, particularly of earthquakes or artificially induced energy for the exploration of underlying rock formations and the interior of the Earth.
2. Any of various devices for measuring and recording the vibrations and intensities of earthquakes.
3. An instrument for recording automatically the phenomena of earthquakes.

A seismograph, or seismometer, is an instrument used to detect and record earthquakes. Generally, it consists of a mass attached to a fixed base. During an earthquake, the base moves and the mass does not. The motion of the base with respect to the mass is commonly transformed into an electrical voltage. The electrical voltage is recorded on paper, magnetic tape, or another recording medium.

This record is proportional to the motion of the seismometer mass relative to the earth, but it can be mathematically converted to a record of the absolute motion of the ground.

The term seismograph generally refers to the seismometer and its recording device as a single unit.