To further clarify: The UK has the distinct titles "reader" and "lecturer" before "professor." In the US, if you have a PhD and you're teaching a college class, you're called a professor, even if you're an adjunct (not tenure track). The tenure track titles are "assistant professor", "associate professor" and then just "professor." Informally, we tend to use "professor" to refer to all of them, and it's common for people to say "Professor Doe" even if the person is an assistant professor.