HMI units, also known as Hydrargyrum medium-arc iodide, are the trademark name of Osram's metal-halide gas discharge medium arc-length lamp. Hydrargyrum is Latin for "liquid silver", abbreviated to Hg.
Unlike incandescent- lighting units, can output closer to 6000K, which is similar to the color of light emitted by the sun. Sun-like light is extremely important when trying to mimic sunlight, or to complement sunlight when shooting outside.
Note that HMI lamps should only be operated by professionals, as their ignition voltage can reach up to 70,000 V, and their ultra- violet light can cause retinal damage and severe burns without the UV safety glass cover.