... is (to quote Wikipedia) "a genus of about 500 species of flowering plants in the family Ranunculaceae. Members of the genus include the buttercups, spearworts and water crowfoots."
The celandines belong to the same family (Ranunculaceae - known in English as the buttercup family), and used to be classified in the same genus as the buttercup (Ranunculus). They are now afforded a genus of their own; the lesser celandine, which used to have the Latin name Ranunculus ficaria, is now known scientifically as Ficaria verna.
The full scientific name of the common or meadow buttercup is Ranunculus acris.
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