適合率とは取り出した記事のうち、どの程度の記事が検索条件に合っているかを示す指標です。

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Precision rate is an indicator showing what proportion of the retrieved articles match the search criteria.

Literal

Precision-rate [topic-とは] retrieved articles [possessive-の] among, what degree [possessive-の] articles [subject-が] search-criteria [to-に] matching [question-か] [object-を] showing indicator [copula-です].

適合率 ('precision rate') is information retrieval terminology. とは is the definition-framing pattern ('as for what X is...'). 取り出した記事のうち ('among the retrieved articles') uses うち for 'among.' どの程度 ('to what degree') is the interrogative within the definition. 指標 (しひょう, 'indicator, metric') closes the definition. A textbook definition sentence.