imperfect tense vs present imperfect

imperfect tense

noun
  • A tense used to describe a past action that is ongoing, incomplete or continuous, or coincident with another action. 

present imperfect

noun
  • A grammatical tense which presents the action in the present as continuous, not yet over 

How often have the words imperfect tense and present imperfect occurred in a corpus of books? (source: Google Ngram Viewer )