This is the 452nd most frequent Ukrainian word.
одна
"One" (feminine).
The word 'одна' is used as a numeral indicating quantity.
Одна книга лежить на столі.
One book is lying on the table.
Here, 'одна' is used as a pronoun meaning 'one' from a selection.
Це одна з найважливіших тем у класі.
This is one of the most important topics in the class.
In this sentence, 'одна' functions as an adjective meaning 'alone' or 'by oneself'.