An initialism for Ceramic Metal Halide. CMH’s were introduced to compete for market share with the then ubiquitous High-Pressure Sodium grow light. Although they don’t outperform HPS lighting as far as yields go, they’re still a decentlight
and can bring in a respectable harvest. These lights weren’t popular to begin with, and their popularity has only waned in the face of HPS’, LEDs, and LECs.