Apparently it is Language Model, as mentioned in the announcement of NotebookLM in 2023 [0].

> Today we’re beginning to roll out Project Tailwind with its new name: NotebookLM, an experimental offering from Google Labs. It’s our endeavor to reimagine what notetaking software might look like if you designed it from scratch knowing that you would have a powerful language model at its core: hence the LM.

It's funny how similar that article's intro is to today's announcement.

[0] https://blog.google/innovation-and-ai/technology/ai/notebook...