This is the 3986th most frequent Spanish word.
biología
Biología: The scientific study of living organisms and their processes.
In this sentence, 'biología' is used as the subject to denote the academic discipline that systematically investigates life forms and their activities.
La biología estudia los seres vivos y sus procesos vitales.
Biology studies living beings and their vital processes.
Here, 'biología' is complemented by 'marina' to specify a particular branch or focus within the field of biology.
Mi hermana está leyendo un libro sobre biología marina.
My sister is reading a book about marine biology.
The term 'biología' is used to indicate a potential career or field of study chosen by Juan.