Break junctions refer to a type of experimental setup used in the field of nanotechnology and molecular electronics to study the electrical properties of individual molecules or nanoscale materials. In a break junction experiment, two metal electrodes are brought very close together and then separated, creating a small gap or "junction" between them. This gap can be adjusted to a nanometer scale, allowing researchers to study the conductance of individual molecules or nanomaterials as they pass through the junction. Break junctions are used to investigate the electrical transport properties, quantum effects, and electronic structure of nanoscale materials, and have potential applications in the development of molecular electronic devices and sensors.