What is the difference of frequency and probability of occurrence?

I'm reading lots of statistical anomaly detection articles nowadays. Some of them use frequency of some feature for detecting anomalies on the new trace.

But, something gets my attention in some articles. Authors use "probability of occurrence" usage than just say frequency. Are they doing this for implying some property of that data set or data? Or frequency is another thing and I don't know the difference?

From [1]:

Using the observations in Section III.B, we develop a technique, called Kernel States Modeling (KSM), to automatically detect anomalies using the probability of occurrences of states in traces.

[1] Murtaza, Syed & Khreich, Wael & Hamou-Lhadj, Abdelwahab & Couture, Mario. (2013). A host-based anomaly detection approach by representing system calls as states of kernel modules. 2013 IEEE 24th International Symposium on Software Reliability Engineering, ISSRE 2013. 431-440. 10.1109/ISSRE.2013.6698896.

• Not sure if I understand your confusion. If you measure frequency of an event over a sample and divide it by the number of samples you get the empirical probability of the event: en.wikipedia.org/wiki/Empirical_probability
– Bar
Commented Oct 24, 2018 at 13:02
• You understood it right. Thanks. I must confuse because mostly I use the physical meaning of frequency.
– user224633
Commented Oct 24, 2018 at 13:18
• A useful question to ask in this regard is "have you ever directly observed a probability?" The answer ought to be no, because a probability is a model construct. But "have you ever directly observed a frequency" ought to have a different answer.
– whuber
Commented Oct 24, 2018 at 19:05