TextExpander is forcing me to enable access for assistive devices. How, and what is that?

For privacy reasons, OS X does not allow third-party applications (such as TextExpander) to log keystrokes, unless access for assistive devices is turned on.

In OS X 10.8 and earlier:

1. Go to the System Preferences and activate the Universal Access preference pane, the one with the little person in front of a blue background. Once you're there, check the box "Enable for assistive devices."

In OS X 10.9 and later:

1. Go to System Preferences -> Security & Privacy -> Privacy -> Accessibility -> TextExpander.