From the Chapter 3, Managing Dependencies, from the book Practical Object-Oriented Design in Ruby, by Sandi Metz:

An object has a dependency when it knows:

• The name of another class. […]
• The name of a message that it intends to send to someone other than self. […]
• The arguments that a message requires. […]
• The order of those arguments. […]

If an object knows any of these facts about another object, it has dependencies to the other.

This is not to say that having dependencies to others is bad, as

A single object cannot know everything, so inevitably it will have to talk to another object. Chapter 3, Managing Dependencies, Introduction

For this latter purpose, there is the section “Writing loosely coupled code”