What does "On-base percentage" mean?

What's "define" mean here? It's not like definition of a word.

You're right! It is different from a "definition of a word." Here, "define" means to draw attention to or to make distinct with a clear outline. Ex: I'll define the type by going around it with a permanent marker. Ex: You can define the eyeshadow by adding a few details with the liquid eyeliner. Ex: The sign isn't very defined.

Is there any position rule for locating "at least" in the sentence?

There is not really any rule for putting "at least" in a sentence, although it usually comes after the verb since it is an adverb of frequency. However, where you choose to put "at least" after the verb mostly depends on your personal choice and does not change the meaning of the sentence at all. You can certainly put it at the end of a sentence if you want. Ex: You should eat one cup of fruit per day, at least. Ex: I have read at least a hundred books.

What's the main difference between a "fibber", a "liar", and a "fraud?"

"Fibber" and "liar" are sometimes used interchangeably. The difference is that a "fibber" tells lies that are small and not very important, whereas a "liar" lies about big and small things. Out of the two, "liar" is more common to use. A "fraud" is a deceptive crime or doing for personal gain. As a person, it's someone who deceives others by how they present themselves or what they say is true, usually to do with accomplishments, qualifications, or qualities. Ex: I'm watching a show about someone who becomes a lawyer but he's actually a fraud since he doesn't have a law degree. Ex: I told my parents a small fib so I could come to this party. I'm a fibber. Ex: She's a liar. She lied about her qualifications on her CV. That could even be considered fraud.

Is "get shot into space" a common expression?

No, "shot into space" is not a very common expression. It is only really used in the context of launching a rocket into outer space. In this case, however, it is used to compare the rollercoaster to a rocket which gets shot into space. Ex: The rocket was shot into space after the countdown.

What's "diminish in" mean?

Actually, "diminish" and "in" function separately from each other here! "Diminish" is a verb that means to make or become less, and the phrase "in any way" is quite a common phrase used in negative sentences that means under any circumstances or conditions. Ex: You're not responsible for my choices in any way. Ex: John was trying to diminish my achievement by criticizing me. Ex: I don't want to diminish your achievements in any way.