Definition from Wiktionary, the free dictionary
- Any device that activates or deactivates the components of a machine or system according to a preplanned sequence (as in a washing machine, or central heating system).
- A device or system that orders and/or modifies digitally stored music and sound for playback.
- a MIDI sequencer
- A device for determining the sequence of monomers in a polymer, especially amino acids in protein, or bases in DNA.
- A control device to increase/decrease the capacity of plant that is enabled to act in a specific order or sequence.
Derived terms 
- boiler sequencer - enables/disables additional boilers in large buildings as the demand for heat increases due external temperature compensation
- pump rotation sequencer - enables a "lead" pump while keeping a second pump as "standby" (in the event of lead failure), but periodically changes the sequence to ensure even maintenance.
- DNA sequencer
- protein sequencer
- state sequencer