N3
み
~ness, Used to indicate conditions or nature
大人になってから将棋の楽しみが分かってきた。
Since becoming an adult, I've come to understand what makes shogi fun.
Explanation
「み」 attaches to the stem of an adjective and turns it into a noun (drop 「い」 from i-adjectives; na-adjectives attach directly). It does not give a measured degree. It gives a feeling you can sense directly, a taste, touch or mood: 「深み」 (depth, richness), 「重み」 (weight, gravity), 「悲しみ」 (sadness), 「温かみ」 (human warmth). Only a limited, fixed set of adjectives take it.
Mixed up with
「さ」 versus 「み」 is a classic exam point. 「さ」 is a measurable, objective degree and almost any adjective takes it: 「深さ10メートル」, 「重さ5キロ」. 「み」 is a felt quality or state: 「文章の深み」, 「赤み」. You cannot say 「×深み10メートル」.
Only some adjectives of feeling, mind or colour take 「み」. Do not invent new ones.