Which novel by Robert A. Heinlein introduced the term 'grok'?

Which novel by Robert A. Heinlein introduced the term “grok”? Stranger in a Strange Land. Heinlein coined “grok” in this 1961 science-fiction novel, where it is presented as a Martian word with no exact English equivalent.

In the story, grokking involves understanding something so completely and intuitively that the observer becomes deeply identified with it. The word is linked to Martian ideas about water, life, empathy, and shared reality. Its meaning is broader than simply “understand”: it suggests sympathetic, almost total comprehension.

The novel follows Valentine Michael Smith, a human raised on Mars who returns to Earth as a young adult. His unfamiliar Martian worldview exposes assumptions in human religion, sexuality, law, and social organization. The term became influential beyond fiction and entered counterculture and technical communities, where “grok” often means to understand a system thoroughly. It is sometimes mistakenly attributed to another Heinlein novel, but Stranger in a Strange Land is the source.