The full-spectrum LED grow light is the newest evolution of an already confusing term. Originally, full-spectrum light described the only real full-spectrum light source, the sun.
Over time, the term began to take on other characteristics of sunlight. The commercial lighting industry began using the name “full-spectrum” to sell lights that produced a Color Rendering Index (CRI) over 90. Humans perceive colors more accurately under light sources with a CRI over 90, much like how we see colors in our natural world under daylight. This was a beneficial feature for human environments such as offices, outdoor spaces, and others.