To North American speakers of English, the word to use is “math”, as in “I majored in math”, and “maths” would sound wrong. Speakers of British English, however, would always say “maths”, as in “I took a degree in maths”. They would never say “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, most people would 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.
It’s sometimes surprising how much argument and disagreement small differences such as that single letter can make. Readers in the UK, for example, sometimes get very upset if someone writes “math” rather than “maths”. No doubt the reverse is true in the US. In practice, it’s simply worth being aware of the geographical differences so that you can use the correct form of the word in your writing.
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