… unlike the lecture about the configuration file, where it is defined as:
Yet Another Markup Language.
Probably best to get it as per YAML 
$ curl -s https://yaml.org | ack -m1 yaml_key
YAML:
YAML Ain’t Markup Language
Hey @Ceri_18853
Just FYI these videos were likely recorded before they changed YAML to the new meaning.
From the Wiki
Originally YAML was said to mean Yet Another Markup Language ,[12] referencing its purpose as a markup language with the yet another construct, but it was then repurposed as YAML Ain’t Markup Language , a recursive acronym, to distinguish its purpose as data-oriented, rather than document markup.
Perhaps if/when the video course is updated it will be changed to the new acronym.
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Probably correct 