Learn English – What’s the term for “hearing” words when you read or think

This is multiple level thinking, which is a term common in the world of poker.

From https://www.google.co.uk/amp/s/assaadmouawad.wordpress.com/2011/10/24/multiple-level-psychology/amp/

Multiple level thinking is a term commonly associated with deepstack poker which refers to a players ability to analyse their opponents actions aswell as their own and think on a number of different levels in order to establish a range of poker hands your opponent could be holding and a range of reasons for their corresponding actions. The table image of you and your opponents plays a major part in how you approach the deeper levels of thinking, the poker becomes more about psychology than the mechanics of the game.

There are 5 levels to multiple level thinking:
Level 0 What do I have?
Level 1 What does my opponent have?
Level 2 What does my opponent think I have?
Level 3 What does my opponent think I think he has?
Level 4 What does my opponent think I think he thinks I have?

Cambridge dictionary defines overhear as

to hear what other people are saying without intending to and without their knowledge

(link)

Oxford dictionary defines the word as

Hear (someone or something) without meaning to or without the knowledge of the speaker.

(link)

Lots of times you "overhear" someone by accident and then it's your choice whether to keep listening intentionally or not to do so. When you begin to intentionally listen you are not really "overhearing" someone or something, rather you're deliberately listening to someone, probably without their knowledge–and that becomes eavesdropping.

You've shown that the American Heritage dictionary does not mention anything about the hearer's intentions but the two dictionaries I cite do so, with Cambridge including the 'accidental part" in the meaning. Two or three other dictionaries–including the Oxford English Dictionary, Merriam-Webster, Collins–do not mention the hearer's intention, but this is not as full a definition as the two I've included, and with which other speakers agree in comments.