Is “math” or “maths” the correct word to use as the shortened or colloquial form of the word “mathematics”? The answer is that it depends on where you are.
Because we follow British English, we would always say “maths”, as in “I took a degree in maths”. However, your North American friends would be using “math”, as in “I majored in math”.
There are logical arguments for both spellings. The word “mathematics” can be considered as a singular and as a plural noun. Both the Oxford and the Merriam-Webster dictionaries say the word is plural – hence the ‘s’ on the end – but also that it is usually used as if it was a singular noun.
So, you say “mathematics is my best subject” and not “mathematics are my best subject”. The shortened form “maths”, then, makes sense because the word is still a plural noun and so should still have the “s” on the end. On the other hand, it could be argued, “math” makes sense because it seems wrong to remove the letters “ematic” from the middle of the word and leave the final “s”.
There are a number of other plural nouns that are used as if they were singular – for example economics, ethics, politics, gymnastics, measles and dominoes. These words, however, are not habitually shortened, making math/maths rather an unusual word.
In practice, it’s simply worth being aware of the geographical differences – say, if you were to go on an exchange programme in the US – so that you can use the correct form of the word in your writing.