I have a friend who, as a child, loved to noodle around on the piano (improvisation) and especially liked the sound of just playing on the black keys. She had probably heard a few musical terms tossed around, and came to believe that sound she liked was because the black keys were the "minor" keys. Of course, the important thing is that she was making music and having fun. That should certainly take priority over knowing the correct terminology. However, if you ever hope to communicate with others about music (let's say you want to play in a group of other musicians), it will behoove you to use the correct terms so you can make yourself understood. In addition, operating under a false understanding of what you are doing can limit you from other things you may wish to play in the future. (The black keys are not "minor;" the sound she created was based on a pentatonic -- 5-tone -- scale because of the 5 black keys, which has a distinct "flavor" because of it's internal relationships.)
Almost every new student who comes to me for lessons has some degree of confusion, or out-and-out misunderstanding, of what major and minor mean in music. It's understandable they'd be confused if they are totally new to playing an instrument. But students with prior experience are also confused, because their teachers never thought to explain these terms correctly. Often, even the teacher doesn't know!
The word "major" means larger and the word "minor" means smaller. Both are originally from Latin. Ursa major is the larger constellation; ursa minor is the smaller one. If you have a major problem, it's a big problem; if you have a minor problem, it's a small one. We often conflate minor with meaning less important, due to being smaller. However, the true meaning is simply smaller. And so it is in music as well. In music, minor is never "less important."
If you've read the previous post, you'll see that major and minor never refer to a single note. A single note can never be large or small.
Major and minor most frequently are used to describe an interval, that is, the "distance" between two tones. Within our musical scales, there are four intervals that have two "sizes," small and large. They are seconds, thirds, sixths, and sevenths. C to D-flat is a minor second (also referred to as a half step), and C to D is a major second (also called a whole step). C to E-flat is a minor third, C to E a major third. C to A-flat is a minor sixth, C to A a major sixth. C to B-flat is a minor seventh, C to B a major seventh. In all cases, the minor interval is the smaller distance, the major the larger. There are 3 intervals which do not have two sizes: fourths, fifths, and octaves. They are called "perfect."
A major triad (3-note-chord) is so called because its first interval (starting from the root) is a major third. A minor triad is so called because its first interval is a minor third. It's really that simple. One very common misconception is that major means "happy" and minor means "sad" (or dark, somber, etc.). I would guess that least 90% of non-musicians (and brand-new students) I talk to think this is the case. However, play a major 7th, for example, and you'll find you don't get the "happy sound" you have associated with major. Happy and sad may be the emotional responses we have to those specific sounds, but they are not the meaning of the terms. (Interestingly, it appears to be universal that we experience a major chord to sound happier and a minor chord to sound darker, sadder. No one ever seems to feel the opposite. However, there is no explanation as to why we humans respond this way. It's another cosmic mystery of music!) So you mustn't confuse the actual meaning of the terms with the way we respond to them.
Major and minor can also refer to scales. Major was origianlly called Ionian and minor was originally Aeolian (from Greek). Ionian and Aeolian were two of seven possible scales (that we know of). I have read many articles about this but there doesn't seem to be clear answer as to why the names were changed. However, in Western Classical music, the "major" scale and the "minor" scales became the preferred sounds. The other five scales are now usually referred to as "modes," however, they are scales. See my post titled Modes if you are interested to learn more.
So why was Ionian renamed to major and Aeolian renamed to minor? You can look it up on Google, or use AI, and there are many different reasons given, none of which I find completely satisfactory. Since it happened hundreds of years ago, there doesn't seem to be any documentation as to why this change was made. My theory is this; in a major scale, starting from the tonic (first note) the intervals of seconds, thirds, sixths and seventh, are all "major," that is the "larger" ones. In a minor scale, thirds sixths and sevenths are all "minor," that is the smaller ones. The "perfect" intervals as well as the second remain the same in both.
Again, people hear a major scale and think it sounds happy, and minor scale not-so-happy. But remember, the sound you hear in each scale is a function of its relationships of the tones to each other. In music, the relationship is everything.
A piece of music in a major key is based on a major scale; a piece in minor is based on a minor scale. A piece of any length or complexity will not stay in the same key for the whole piece, but is identified by the scale that the piece begins in, and likely ends in.
Since the terms major and minor are used to descibe four musical entities -- the interval, the chord, the scale, and an entire piece -- it's easy to see why there is so much confusion. I hope this has cleared it up.