A matter component with relativistic motion during an important part of structure formation. Its free streaming suppresses small-scale perturbations. Making a light thermal neutrino Majorana rather than Dirac does not by itself turn this component into cold dark matter.
Hot dark matter (HDM) is a theoretical form of dark matter that is characterized by particles that travel at relativistic speeds, meaning they move close to the speed of light. This contrasts with cold dark matter (CDM), which consists of particles that move slowly compared to the speed of light. The concept of hot dark matter primarily includes lightweight particles, such as neutrinos.
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