The word "default" is used a bit weirdly in Chrome. The "default search engines" are ones intended to provide search to the omni bar; as opposed to other search

The word "default" is used a bit weirdly in Chrome. The "default search engines" are ones intended to provide search to the omni bar; as opposed to other search engines which Chrome just picks up passively when you browse the web (often these are suggested when you start typing).