Ethos is a rhetorical appeal that refers to the credibility, character, or ethical appeal of the speaker or writer. It is one of the three modes of persuasion identified by Aristotle, alongside pathos (appeal to emotion) and logos (appeal to logic and reason). Ethos is used to establish trust and authority, persuading an audience by demonstrating that the speaker or writer is knowledgeable, trustworthy, and has the moral integrity to speak on the subject at hand.

Circular symmetry, often referred to as radial symmetry, is a type of symmetry where an object or shape appears the same when rotated around a central point. In other words, if you were to rotate the object through any angle about that central point, it would look unchanged. In the context of two-dimensional shapes, examples of circular symmetry include circles, wheels, and starfish. In three dimensions, objects like spheres and some types of flower arrangements exhibit circular symmetry.

The crystal system is a classification of crystals based on their internal symmetry and geometric arrangement. In crystallography, scientists categorize crystals into seven distinct systems according to their unit cells—the smallest repeating unit that reflects the symmetry and structure of the entire crystal. The seven crystal systems are: 1. **Cubical (or Isometric)**: Characterized by three equal axes at right angles to each other. Example: salt (sodium chloride).

The term "Einstein Group" doesn't refer to a widely recognized concept in academia or other fields as of my last update in October 2023. However, it could relate to several different contexts depending on what you're referencing: 1. **Scientific Community**: It might refer to a group of physicists or researchers who focus on topics related to Einstein's theories, especially in the realms of relativity or quantum mechanics.

Facial symmetry refers to the degree to which one side of a person's face is a mirror image of the other side. In a perfectly symmetrical face, corresponding features (such as eyes, eyebrows, lips, and jawline) match in size, shape, and position on both sides. However, most human faces are not perfectly symmetrical; slight asymmetries are common and can even contribute to an individual's uniqueness and attractiveness.