【Fidelity】 , 【allegiance】 ,  【fealty】 ,  【loyalty】 ,  【devotion】 ,  【piety】  denote faithfulness to something to which one is bound by a pledge or duty.

【Fidelity】 implies strict adherence to what is a matter of faith or of keeping faith; it presupposes an obligation, sometimes natural, sometimes imposed as a trust, and sometimes voluntarily accepted or chosen. Sometimes, even when unqualified, it implies marital faithfulness.  Sometimes it implies faithfulness to the original (as in representation, portrayal, or quotation).

【Allegiance】 implies adherence to something objective which one serves or follows as a vassal follows his lord and which demands unswerving 【fidelity】 when conflicting obligations dispute its preeminence.

【Fealty】 , like  【allegiance】 , implies a supreme obligation to be faithful, but unlike the latter it stresses the compelling power of one's sense of duty or of consciousness of one's pledged word.

【Loyalty】  may imply more emotion and closer personal attachment than either  【fidelity】  or  【fealty】 ; it usually connotes steadfastness, sometimes in the face of attempts to alienate one's affections or of a temptation to ignore or renounce one's obligation, but in some contexts it may be taken to imply no more than absence of anything treasonable or subversive.

Unlike  【fidelity】 ,  【loyalty】 sometimes suggests a personal and emotional attachment often without rational basis.

【Devotion】 stresses zeal in service often amounting to self-dedication; it usually also implies ardent attachment.

【Piety】 emphasizes 【fidelity】 to obligations regarded as natural or fundamental (as reverence for one's parents, one's race, one's traditions, one's country, or one's God) and observance of all the duties which such 【fidelity】 requires.