How is arc defined in TikZ?
In TikZ, an arc is simply a curved path in the shape of a portion of a circle. You can draw an arc using the `arc` or `arc to` command. These commands take three arguments: the center of the circle, the starting and ending angles, and the radius.

Here's an example of how you can use the `arc` command to draw an arc in TikZ:
```
\documentclass{article}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}[scale=3]
I couldn't a decent definition a TikZ book or PGF manual. Can someone please explain?

In TikZ, the `arc` command doesn't its own definition -- it instead makes use of the `pic` mechanism to draw arcs in a parametric fashion. Here's a brief explanation:

1. First, it's important to understand that TikZ and PGF are not just standalone packages, but rather libraries that can be used with LaTeX or other TeX engines. They provide a large set of features for creating graphics, including commands