A population could be any subjects of interest in your study:
For example if your study is about ecosystem then the population is the total number of beings in that particularly specified area of study.
Sample population of study taken from: http://upetd.up.ac.za/thesis/available/etd-10132006-135624/unrestricted/03chapter3.pdf
POPULATION OF THE STUDY
The target population for this research defined to include the music teachers and
music educators in Nigeria, while the accessible population is the music teachers
and music educators in Anambra State, since these are the music teachers and
music educators within the researcher’s reach.
In this study, the accessible population comprised all the music teachers and music
educators in the 6 educational zones of Anambra State in Nigeria at the primary
school level of education. Table 3 below presents a record of the number of the
music teachers and music educators in the educational zones as at August, 2003.
Table 3: The population of the music teachers’ and music educators’ in the
educational zone of Anambra State in Nigeria
In addition, the music teachers and music educators were considered appropriate
as population of the study area because, as stated in chapter one, they constitute
the dramatis-personae responsible for routine teaching of music in the schools.
Most of them have had several years of music teaching and learning and therefore,
they are in the best position to furnish the researcher with the information needed
to answer the research question of this study.
Another example( taken from www.christian.ac.th)
Title of the study-The Effects of Ice Chips on Reducing Sore Throat after Extubation in Patients Undergone Surgery with General Anesthesia at Sampran Hospital in Nakhon Pathom Province.
Definition of the Population:
The population of this study were pregnant patients, who received caesarean section and general anesthesia, with endotracheal tube. After extubation, the population who had pain score level of sore throat ≥ 4 were selected.