Substrings are essentially a portion of a string that can be extracted out based on their position or pattern. There are several synonyms for the term 'substring', namely - segment, fragment, portion, fraction, partition, slice, etc. 'Segment' refers to a part of a whole, 'fragment' connotes a broken piece of something, 'portion' denotes the division of something into parts, 'fraction' means a part of a whole, particularly when it is a small part. 'Partition' indicates dividing something into parts by a clear boundary, while 'slice' suggests dividing something into thin pieces, often with a sharp instrument. All these terms can be used interchangeably with substrings, depending on the context in which they are being used.