The term 'pursue or chase relentlessly' can be summarized by the word 'HOUND, ' which is a five-letter word. A HOUND is a type of dog that is bred and trained to hunt, track, or retrieve game. Hounds are known for their strong sense of smell and their ability to follow a scent for long distances. They are also known for their tenacity and determination in pursuing their quarry. In other words, hounds are relentless in their pursuit of their target.