which is to say

Which do you say more often?

What situation did you imagine, in which someone would say that?

deWas für eine Situation, in der das gesagt wird, hast du dir vorgestellt? — Pfirsichbaeumchen

My father studies astronomy, which is to say the science of stars.

ruМой отец изучает астрономию, то есть науку о звёздах. — marafon

The vase was empty, which is to say it wasn't filled with anything.

deDie Vase war leer, das heißt mit nichts befüllt. — brauchinet

Have you found the umbrella which you said you had lost the other day?

ruТы нашёл зонтик, который, как ты говорил, ты потерял на днях? — Hellerick