A thumb was a type of finger.

Lepscillians had a massage technique in which the thumbs were pressed against the temples. This made a human go to sleep and feel refreshed when they woke up. (PROSE: Forever Autumn)

The Ralafeans had four thumbs and one finger on each hand and invented the mobile phone before the printing press. (PROSE: Wooden Heart)

After turning into a dinosaur due to the Eleventh Doctor's interference in prehistory, Amy Pond complained that she couldn't lock her phone because she no longer had thumbs. (COMIC: Short-Change)

The "thumbs up" gesture was used to indicate approval. It was also used as an emoji: 👍. (TV: Smile)

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