.dependencies

A file to define environment dependencies for applications

Example

In a file named .dependencies at the root of your project:

ruby: "~> 2.3", file: "Gemfile"
postgres: "~> 9.3"
python: "~> 2.7", file: "requirements.txt"
node: "~> 5.5", file: "package.json"

Take the line ruby: "~> 2.3", file: "Gemfile". Let's break it down.

ruby is the name of the system-level software being dependend on by your program
"~> 2.3" defines the version constraint for the ruby dependency, ~> means "loose" which is equivalent to saying "I want any version including 9.3 and above but below 9.4".
file is an optional key that allows you to specify where the system-level dependency can find its own dependencies, and in Ruby programs usually document their dependencies using a Bundler Gemfile.