This is the 3165th most frequent Spanish word.
xxi
"XXI" is not a Spanish word but the Roman numeral for 21.
Used as a numeral designating the century.
Vivimos en el siglo XXI.
We live in the 21st century.
Used in a formal title to specify a sequence number.
El evento tendrá lugar en el XXI Congreso Internacional.
The event will take place at the 21st International Congress.
Refers to a time period introducing modernity.