Antonyms of adj 21st

1 sense of 21st

Sense 1
twenty-first, 21st -- (coming next after the twentieth in position)

INDIRECT (VIA ordinal) -> cardinal -- (being or denoting a numerical quantity but not order; "cardinal numbers")

Similarity of adj 21st

1 sense of 21st

Sense 1
twenty-first, 21st -- (coming next after the twentieth in position)
       => ordinal (vs. cardinal) -- (being or denoting a numerical order in a series; "ordinal numbers"; "held an ordinal rank of seventh")