It doesn’t necessarily mean you can’t claim it back. Twitter’s inactive user policy allows people to reclaim inactive accounts after 6 months of non-tweeting or no activity, or if you own the main domain name (.com)
i.e. if you own Omnisity.com and someone already has taken the Twitter username @omnisity there is a very good chance you can file an impersonation claim and get @omnisity for yourself. This will only work if the account is truly trying to represent your brand/company and/or the account has been inactive for over 6 months.
Find the following, which is a step by step guide on how to claim an inactive Twitter Username
2. Make the selection for ‘I am being impersonated’.
3. Follow all the steps and fill the form, providing Twitter with the information requested.
4. Complete the form, giving a detailed description of why you think the twitter handle should be released. This is where you should tell them you own the .com and any other entities related directly to the username. (You will likely have to prove it by emailing them from the domain). If you have any registered trademarks etc for the name, you can tell them this as well.
5. Within a day or two you should get a response from Twitter support and have an answer.