A genre of games inspired by an aesthetic more than anything else, steampunk games are distinguished from others through their use of technology that is based upon steam power. Sometimes but not always set in a world that might resemble the Victorian period, steampunk games hold to no concrete rules other than that of using technology inspired by the steam power and the general look and feel of the British Empire during the reign of Queen Victoria.
In some ways, the games are inspired by the “fantasy” of that time period so you will see such things as airships though this isn’t a necessity. Because of its broad application, you can see steampunk elements in games like Bioshock Infinite, Final Fantasy VI, and even titles like Bloodborne which meld the gothic Victorian with a steampunk understanding of technology.
Fashions, architecture, and technology in steampunk games tend to be influenced by the Victorian period but this isn’t a hard and fast rule – really the only necessity is that technology must be, on the whole, driven by fire and steam tech.
And it really doesn’t matter if the applications of this technology are realistic, such as in the case of submersibles or even vessels that go into space. This is where the aesthetic of a steampunk game comes heavily into play. The game can have outlandish conceits but it has to hew more or less to the idea that the main source of power for all things associated with technological advancement is derived from some form of steam power.