These words describe persons or things that do not let up or swerve from their course out of any feeling of compassion. Inexorable is the most formal word. It may be used of unyielding, unappeasable persons: an inexorable foe; his inexorable will. But it is more often applied to inhuman or objective forces that cannot be avoided, stopped or changed: Oedipus could not escape his inexorable fate.

Remorseless , relentless and unrelenting are closest in meaning to inexorable . All imply a powerful, driving force and unceasing progress towards a goal: driving himself at a relentless pace; to apply relentless pressure; an unrelenting fight against crime. But remorseless is the most chilling word of the three, as it may indicate utter unresponsiveness to human values: the remorseless progress of a fatal disease; to remorseless ticking of a time bomb. Relentless is sometimes close to remorseless in this sense, but it is usually less severe and is not always damning by any means. In general use, it may apply to anyone or anything that shows no mercy and cannot be stopped by entreaty or appeal: a relentless prosecutor; the relentless passing of time. Specifically, relentless is sometimes used as in intensification of tireless. As such, it can imply a refusal to yield under punishing pressure: He pressed on relentlessly , trying to meet the deadline. Or it may focus on an implacable persistence in hounding another: to relentless Javert pursuing Jean Valjean in Les Misérables. Unrelenting may be less extreme in its implications, sometimes suggesting a proper rather than a cruel refusal to relent. An unrelenting person may not be impervious to entreaty, but he is able to resist it. He stands firm and does not give in when he might be expected or tempted to do so: The father handled his disobedient son with unrelenting firmness. Unrelenting may also suggest the unshakable resolve and fierce dedication of someone firmly committed to a cause and not to be turned aside by obstacles: an unrelenting crusader for women’s rights.

The remaining words imply unfeeling harshness or cruelty. Merciless , line inexorable , is often used of impersonal forces: subjected to a merci less bombardment of noise; the merciless glare of the noonday sun. when it involves the agency of persons, it usually focuses on their instruments or acts rather than on their feelings: the merciless eye of the camera; a merciless beating.

• A merciless mob of reporters descended on the bereaved family.

Pitiless , by contrast, is more often used to describe human beings as such. It suggests a cold, steel-hard quality: her pitiless eye; a pitiless tyrant. Inexorable , relentless and pitiless may all imply a refusal or inability to make or allow the slightest concession: inexorable logic; relentless reason; the pitiless efficiency of a machine.

Ruthless conveys a greater degree of harshness than any of these other words, for it focuses on a fierce refusal to give any quarter. It is applied to persons who are so unsparing as to be cruel, driving forward single-mindedly, and sometimes savagely, with a brutal disregard for those they attack or exploit: a ruthless slave driver; ruthless in his criticism; ruthlessly frank. Applied to personal expression or action, ruthless may mean harsh, brutal, unsparing, cold-blooded or cut-throat: ruthless methods; ruthless competition in the tournament finals.