A split in phonology is where a once identical phoneme diverges in different instances. A merger is the opposite: where two (or more) phonemes merge and become indistinguishable. In English, this happens most often with vowels, although not exclusively. See phonemic differentiation for more information.
Due to the wide geographic distribution of the English language and the number and variety of speakers, there are phonemic pairs which are distinguished in some accents and varieties, but considered identical in others.