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A joule thief is a minimalist self-oscillating voltage booster that is small, low-cost, and easy to build, typically used for driving small loads, such as driving an LED using a 1.5 volt battery. It can use nearly all of the energy in a single-cell electric battery, even far below the voltage where other circuits consider the battery fully discharged (or "dead"); hence the name, which suggests the notion that the circuit is "stealing" energy or "joules" from the source – the term is a pun on "jewel thief".
The circuit is a variant of the blocking oscillator that forms an unregulated voltage boost converter.

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