The Japanese language is very flexible with adjectives. The sa-form allows to use an i-adjective as a noun or in other words to add ~ness to it (e.g. useful => usefulness, powerful => powerfulness, etc.)

It is very easy to build. Just replace “i” at the end of the word with a “sa”. So 楽しい (tanoshii, enjoyable) becomes joy as 楽しさ (tanoshisa). Here are a few more examples to make it clear.

Example sentences:

日本のお寺の美しさが好きです。

nihon no otera no utsukushisa ga suki desu.

I like the beauty of Japanese temples.

Related articles:

• The sa-form of i-adjectives
• The kunakatta-form of i-adjectives