# What do the terms 'Sample' and 'Sampling' mean in the discussion of Pattern Recognition and Machine Learning?

The tag sampling tells me that,

Creating samples from a well-specified population using a probabilistic method and/or producing random numbers from a specified distribution.

Now, the question becomes: what is population then?

Suppose, I am implementing a classifier.

I have a set of training data in the file train.txt, and a set of test data in the file test.txt.

What element can be termed as 'Sample', and what activity can be termed as 'Sampling' in this case?