I haven't heard "instant dinners." "TV dinner" is an older phrase, but still recognizable to me. Maybe not to the kids today.
I would tend to call them "frozen dinners." You pop them in the microwave for a few minutes, and you're good to go.
"Instant" tends to be used only for things like a quick soup that you "just add hot water" to. Or "instant rice" that is ready in 5 minutes instead of 20.
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