major constituent
If a fruit smoothie is mostly banana with a sprinkle of cinnamon, the banana is the big, obvious part — what most of the smoothie is. A major constituent is that big part of a sample: a component present in a large proportion, making up a substantial fraction of the whole.
A major constituent is a component present at high concentration, conventionally meaning roughly 1 percent or more of the sample (some define it as 10 percent or more). It contrasts with minor constituents (in between) and trace constituents (the very small amounts). The word describes how abundant something is, not whether it is the analyte you care about.
It matters because how much of something is present shapes which methods make sense: measuring a major constituent often allows simpler, classical techniques, while a trace constituent demands the most sensitive ones. The caveat is that 'major' is about proportion, not importance — a trace toxin can matter far more to health than the major constituent water it floats in.
In a stainless steel sample, iron and chromium are major constituents (tens of percent each), while trace impurities like sulfur sit at parts per million. The major constituents define what the alloy basically is.
A major constituent makes up a large fraction of the sample.
Major, minor, and trace are bands of concentration with no universally fixed cutoffs. They tell you how to plan the measurement, not how important the component is.