imperfective vs past continuous

imperfective

noun

  • The imperfective aspect; a verb having this aspect. 

adj

  • Of, relating to or having the properties of the imperfective aspect. 

past continuous

noun

  • A tense used to describe actions and states continued in the past. 

How often have the words imperfective and past continuous occurred in a corpus of books? (source: Google Ngram Viewer )