Is there any “official” term to describe a study in which subjects were deceived about the real purpose of the research?

I am talking about studies that purposefully hide the real research questions from the subjects so as to prevent demand effects from influencing the results.

To make it more explicit: In my case, I tell people to watch a series of video clips from an online portal to describe the contents. What people don't know is that some of the videos are (purposefully) significantly distorted. I record their reactions to it (e.g. physiogically) — but people still think they're just describing videos.