Is Boston flat or hilly?

Hello there, I'm curious about the geography of Boston. Could you tell me, is Boston known for its flat terrain or is it more hilly? I've heard conflicting opinions on this, so I'm hoping to get a definitive answer. I'm particularly interested in this because I'm planning a trip to the city and I want to make sure I'm prepared for any potential hills I might encounter while exploring. Thanks in advance for your help!