1. soft or smooth, not harsh, especially of sound, flavour or colour.

Frame sentences: I like the mellow tone of your voice.

Opposite: harsh.

related words (thesaurus):

dulcet, tuneful, sweet-sounding, euphonious, melodious, lyric, fruity, mellifluous, warm, smooth, rich, well rounded, mellifluent.

2. Tempered by maturity or experience (of a persons character)

Frame sentences: You are growing mellow with age.

related words:

easy-going, amicable, tolerant, amiable, warm, warm-hearted, afable, sympathetic, good-natured, gentle, gracious, kindly, pleasant, kind-hearted.

3. Mellow mangoes.
opposite: green, unripe

mature, ripe, soft, juicy, lush, tender, sweet, flavorsome, luscious

4. I am in a mellow mood.
opposite: irritable, nasty

jolly, jovial, cheerful, genial, happy, merry.

Mellow as a VERB:

5. Eight years of meditation mellowed him.

related words: calm, relaxed, mature, settle, soften, improve, sweeten.

6. Age has mellowed the concrete.

related words: season, condition, improve age.