noun
proficiency level; degree of skill; learning level
Used to describe how well someone has mastered a skill or subject, often in educational or training contexts.
このテストで英語の習熟度を測ります。
This test measures your English proficiency level.
彼のピアノの習熟度はかなり高い。
His piano skill level is quite high.
習熟 is the verb/noun meaning 'to become proficient' or 'proficiency'; 習熟度 adds the degree/level aspect.
Compound of 習熟 (proficiency) + 度 (degree). The exact historical derivation is uncertain, but the word follows a common pattern for expressing levels of ability.