The domain I want to register is not available, even though it's expired.

all killer.  no filler.

Domains are released after the end of their term if they have not been renewed. Domains that expire at another registrar are subject to their own policies; however, there is usually a grace period, and then a period when the domain is “on hold”- and cannot be registered. You should allow at least 30 days from the time of expiration before you can try registering the domain.

To find out a registrar’s specific release policy, you should contact the registrar that owns the name. You can find out who is the registrar for a specific name by entering the name into the central database search engine at http://www.internic.net/whois.html.