You've left out too much that you can see, but we can't. We've no clue if you're looking at a big quarter-sized power transistor, a tiny smd device the size of a gnat, or anything in-between. And for that matter, are you even sure it's a transistor? Is there any labeling on the circuit board? What's the nature of the device it's in? Approximately how old is it? (That tells us what vintage the components are.)
I'd suggest a picture might be the easiest way to convey that information.