In your dictionary, "mayor" may first pop up as the comparative and superlative form of "grande" meaning "bigger; biggest; older; oldest," but it carries other meanings, too. For example, "al por mayor" means "wholesale." Listen to our new featured song "Aparencias" ("Appearances"), where Héctor Montaner croons:
Si el amor me lo vendieran al mayor,
pagaría lo que fuera por comprarlo.
"If love were sold to me wholesale,
I would pay anything to buy it."
captions 5-6, Héctor Montaner > Aparencias
Note that Montaner takes a little bit of poetic license by skipping "por": In your dictionaries, "vender al por mayor" is the full, correct way to say "sell wholesale."
Elsewhere in new videos, Francisco in Salamanca describes his campus as:
...donde vamos los mayores
"...where we, the elders, attend"
caption 7, Francisco Pérez > Salamanca
At his school, mayores are defined as anyone 55 or older, but mayores can also simply mean "adults." You'll have to listen for the context to discern if mayores refers to ancianos, viejos y/o abuelos -- that is to say, the elderly -- or simply someone who's an adult, or of legal age.
If in doubt about the meaning of mayores (as a description or a comparison), heed this expression:
¡Más respeto con los mayores!
"Be more respectful to your elders (and betters)!"