There isn't a single, universally accepted "guide word" for physics. It's a very broad field, and different aspects of physics could be described by different words.

Here are some possibilities, depending on what you want to emphasize:

General:

* Nature: This is a very broad, encompassing term that emphasizes physics' study of the fundamental workings of the universe.

* Matter and Energy: This captures the core focus of physics on the building blocks and interactions of the universe.

* Motion: This reflects physics' exploration of how things move, change, and interact.

Specific Focus:

* Laws: This emphasizes the search for fundamental rules and principles that govern the universe.

* Measurement: This highlights the importance of precise observation and quantification in physics.

* Experiment: This points to the essential role of experimentation in testing and refining theories.

* Theory: This focuses on the development of explanatory frameworks for understanding physical phenomena.

Ultimately, the best "guide word" for physics depends on your specific purpose and the aspect of physics you want to emphasize.