The definition of honor varied widely between cultures, as did the importance placed upon it. Honor was normally synonymous with dignity and referred to the upholding of one's values. The Klingons were notorious for the emphasis they placed on honor.
This may have been a more abstract way of viewing honor. But, as notions of honor were often very subjective, putting emphasis on clear objects (i.e., victory) made the Klingons able to deduce the honorable conduct (i.e., means used to achieve victory).
Although Vulcans suppressed emotions, they nevertheless had a sense of honor. (ENT: "The Andorian Incident") In 2152, T'Pol, explaining to Captain Jonathan Archer why she was sent by the Vulcan High Command to capture a criminal who escaped her during her time as an agent with the Ministry of Security, stating that they considered it a matter of honor. Archer found this to be a very Vulcan attitude. (ENT: "The Seventh")
Honor was one of the most important components of Klingon philosophy, as their social order depended on an inviolable honor-shame dynamic. However, it was significantly different from most other cultures' definition of the word. For example, Worf once said, "Nothing is more honorable than victory." From this point of view, using a cloaked ship to ambush enemies could be considered honorable if it achieved victory. (DS9: "The Way of the Warrior"; DIS: "The Vulcan Hello")
The notion of honor seemed to be highly relative and open to debate. Each and every Klingon could possibly have his own interpretation of what consisted of an honorable means to achieve an end. Even more, some decisions were paradoxical because two different views of honor were in contradiction. For instance, Worf indicated that it was necessary to challenge Gowron's leadership (because he was presumably acting in a dishonorable way), while General Martok was convinced that it was dishonorable to challenge the leader of the Klingon Empire in the middle of a war. In another case, while Doctor Antaak's deception in claiming to have stabilized Augment DNA, allowing the creation of Klingon Augments, when he and Phlox had merely cured the augment virus might be seen by some as dishonorable, Antaak himself believed that, given the millions of lives this cure would save, his near-certain execution, if caught, would be as honorable a death as a doctor could have. (DS9: "Tacking Into the Wind"; ENT: "Divergence") Hence, dishonor was sometimes a concept which compelled Klingons to action.
Out of all races the Klingons had encountered, the Romulans were widely considered by them to be the most dishonorable; Worf especially hated them and considered them dishonorable due to an attack they had committed on Khitomer and the death of his father. However, he later admitted that they were capable of fighting with honor, when they assisted the USS Enterprise-E in defeating Shinzon's ship, the Scimitar. (TNG: "The Neutral Zone"; Star Trek Nemesis) Klingon opinion on the Federation was widely variable and changed with time, though they were deeply impressed by the sacrifice made by the crew of the USS Enterprise-C, which was destroyed when attempting to defend a Klingon outpost at Narendra III. (TNG: "Yesterday's Enterprise") When tensions reached a high in 2372, Gowron claimed that the Federation did not care for honor, though his opinion changed the following year when Benjamin Sisko's team infiltrated Ty'Gokor to assassinate a Changeling at very high personal risk, leading to a ceasefire between the two sides. (DS9: "The Way of the Warrior", "Apocalypse Rising")
Other races considered dishonorable, by at least some Klingons, include the Breen, (DS9: "'Til Death Do Us Part") the Ferengi, (DS9: "The House of Quark") and the Jem'Hadar. (DS9: "Soldiers of the Empire") On the other hand, the Jem'Hadar did understand the concept of honor, with some even respecting it. The Vorta, however, considered it barbaric. (DS9: "By Inferno's Light") In contrast, the Klingon soldier Leskit claimed to respect the Cardassians as he felt they fought for their people and followed a code, "just like we do." (DS9: "Soldiers of the Empire")
Despite honor being a huge part of Klingon culture and society, there have been known cases of Klingons acting dishonorably. D'Ghor sought to take over the House of Kozak but did so by attacking the House's financial assets, as he did not wish to destroy it through openly challenging it; Grilka was openly disgusted by the very idea of this, and Gowron later discommended him for his actions. (DS9: "The House of Quark") While explaining the hierarchy on Klingon ships, Jadzia Dax mentioned that cowardice was among the conditions under which a subordinate could challenge a superior; Miles O'Brien was surprised at the very idea of Klingons showing cowardice, though Dax stated, "It's been known to happen." (DS9: "Soldiers of the Empire")