As others point out, there are two ways for a company to differ from a startup, which is a new and still-small company that is growing rapidly. One opposite would be a failed company, and the other would be a big corporation that has been around for a while.
Solyndra was a failed startup. Microsoft is a big corporation. A company that is not a startup but is still smaller than a giant like Microsoft is an "established firm" if it does white-collar work or an "existing company" if it sells actual stuff or regular services.