This is the 14372nd most frequent Spanish word.
vigésima
"Vigésima" means "twentieth."
The word "vigésimo" is used as an ordinal number to describe the position of the day within the month.
Hoy es el vigésimo día del mes.
Today is the twentieth day of the month.
"Vigésimo" here specifies the chapter's sequential order in the book.
Estoy leyendo el vigésimo capítulo del libro.
I am reading the twentieth chapter of the book.
In this case, "vigésima" indicates count in the sequence of intersections encountered.