The phrase "major source" refers to something or someone that provides a significant amount or a large portion of something. It denotes that this source plays a crucial role in supplying or causing a particular thing. Full definition
Transportation and fossil fuel industries typically come to mind as major sources of pollution. (sciencedaily.com)
A second major source for skepticism: Evidence of the fish, shrimp and worms don't come from finding DNA snippets from those actual critters. (sciencenewsforstudents.org)
The data suggest wind and solar are quickly becoming major sources of electricity, brushing aside perceptions that they're too expensive to rival traditional fuels. (bloomberg.com)