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There are actually two choices on MLB.com: the yarmulke (pictured above), and the kippah. My preliminary internet research tells me the two terms mean the same thing, but that yarmulke is the Yiddish word, while kippah is the Hebrew word.
This isn't the first time I've seen a baseball logo on yarmulke - there was a Jewish Studies professor at my undergrad institution who was an avid Twins fan, and he used to attend Shabbat services every week wearing a Twins yarmulke.
One week, we reached the part of the service where the service pauses, and people say the names of friends and loved ones who might be sick or injured and in need of prayer, and this professor was sitting directly behind me. All I could hear was him murmuring, "Joe Mauer, Denard Span, Francisco Liriano..."
That day, we all prayed for the future of the Twins lineup, and I did my best to keep a straight face.