A link from your website to another website.
An external or outbound link takes a reader from your website to another site. It can provide an original source, a product manual, an application form or a specialist explanation you do not need to reproduce. The term describes direction: a link leaving your site is outbound, while a link coming from another site to yours is a backlink.
Choose the destination because it helps the reader verify or complete something. Linking to an official record can substantiate a particular statement, but merely linking to an impressive institution does not establish your own credentials or guarantee a ranking benefit. The relationship between the claim and the destination matters more than the prestige of the domain.
Outbound links also require upkeep. The other organization controls its page and can move it, remove it or change its contents. A source that once supported your explanation may no longer do so. Keep especially important references specific enough that an editor can later identify what the reader was supposed to find there.
Takeaway: Link to reliable, relevant sources when they make your explanation more useful. Do not use external links as a substitute for accurately describing your own qualifications or services.
Example
A furniture maker explains that a finish requires a particular care routine. It links to the manufacturer’s actual care instructions, making clear which finish the instructions cover. The link helps a customer look after the purchase; it does not imply that the manufacturer endorses every product the furniture maker sells.
What to check
Check the destination before publishing and during content reviews. Paid or sponsored links need appropriate qualification; ask whoever manages the site to use the relevant link attributes rather than presenting a commercial placement as an ordinary editorial recommendation.
For more detail, see Google link best practices.