What is a wildcard certificate?

A wildcard certificate secures every first-level subdomain of a domain with one certificate, written as *.example.com.au:

• One certificate, many hosts - convenient when subdomains come and go.
• One key, many hosts - which is the trade-off: a compromise of that key affects everything it covers.
• It does not nest - *.example.com.au covers shop.example.com.au but not app.shop.example.com.au.

For a handful of stable subdomains, individual certificates are usually the safer choice. Wildcards earn their place when the list changes often.