vertical bar - chunhualiao/public-docs GitHub Wiki

The vertical bar | is read as “given” or “conditioned on”.

The expression $\pi(a \mid s)$ is typically pronounced as:

"pi of a given s" or "pi of a given s" or "pi of a conditioned on s"

If you're in a more formal setting or talking to people familiar with probability theory or machine learning, you can say:

"pi of a given s" — most common "pi of a conditioned on s" — more formal "pi a given s" — shortened and often used informally

Where:

• $\pi$ is pronounced like the Greek letter "pi" (rhymes with "eye").
• $a$ is the action.
• $s$ is the state.
• The vertical bar $|$ is read as "given" or "conditioned on".

So the whole thing just means: the probability that the policy $\pi$ chooses action $a$ when in state $s$.