If you're open-minded, you're willing to consider new ideas. An open-minded voter doesn't always stick to his or her political party and instead considers the opinions of candidates from both parties. (Finding an open-minded voter is a bit like trying to find a unicorn. Good luck.)
When you've got a serious dilemma — say your two best friends aren't speaking to each other — it's usually helpful to ask an open-minded person for advice. Open-minded people can consider a problem from both sides, without injecting their own opinions or biases, and can often point you in the right direction. Open-minded is first recorded in the 1820s, from the popular figurative meaning of open (think "openhearted").