Infiel vs. desleal
The Spanish words infiel and desleal both essentially mean unfaithful, but they are used in different contexts. Infiel specifically refers to infidelity in romantic relationships, while desleal can refer to disloyalty in any context, including but not limited to romantic relationships.
Infiel
B1The Spanish word infiel is used to describe someone who is unfaithful in a romantic or marital relationship, i.e., someone who cheats on their partner.
Ella descubrió que su esposo era infiel.
(She discovered that her husband was unfaithiful.)
El infiel vive en constante miedo de ser descubierto.
(The unfaithful one lives in constant fear of being discovered.)
Ser infiel es una violación grave de la confianza en una relación.
(Being unfaithful is a serious violation of trust in a relationship.)
Desleal
B2Desleal is a broader term meaning disloyal or unfaithful. It can be used to describe someone who is treacherous or not trustworthy, not only in the context of romantic relationships but also in friendship, business partnerships, and so forth.
El socio desleal fue expulsado de la empresa.
(The disloyal partner was removed from the company.)
Es desleal hacer trampa en un juego.
(It's disloyal to cheat in a game.)
Una acción desleal puede romper una amistad para siempre.
(A disloyal action can break a friendship forever.)
Summary
So, while both infiel and desleal can be translated to mean unfaithful or disloyal, the use of each word depends on the context. You would typically use infiel when referring specifically to cheating in a romantic relationship and use desleal when talking about broader forms of disloyalty.