Bitterness; the quality or degree of being bitter in taste. Can also be used figuratively for bitter experiences.
このコーヒーの苦さがちょうどいい。
The bitterness of this coffee is just right.
苦さを和らげるために砂糖を少し入れた。
I added a little sugar to soften the bitterness.
若い頃の失敗の苦さが、今の糧になっている。
The bitterness of past failures in my youth has become nourishment for me now.