Mandíbula vs. maxilar
In Spanish, while both mandíbula and maxilar refer to the jaw, they are not interchangeable. Mandíbula is the common, everyday term for the lower jaw, while maxilar is a more technical, anatomical term used to distinguish between the upper and lower jawbones.
Mandíbula
A2This word almost always refers to the lower jawbone, the part that moves when you speak or chew. It is the most common and widely understood term for jaw in everyday conversation.
El boxeador recibió un fuerte golpe en la mandíbula.
(The boxer received a strong blow to the jaw.)
Sentí un crujido en la mandíbula al bostezar muy fuerte.
(I felt a crack in my jaw when I yawned really hard.)
El actor es famoso por su mandíbula cuadrada y definida.
(The actor is famous for his square and defined jawline.)
Masticar chicle de forma constante puede causar dolor en la mandíbula.
(Chewing gum constantly can cause pain in the jaw.)
La serpiente puede desencajar su mandíbula para comer presas grandes.
(The snake can dislocate its jaw to eat large prey.)
Maxilar
B2This is a more clinical and anatomical term. It can refer to either the upper jawbone (maxilar superior), which is fixed, or the lower jawbone (maxilar inferior), which is a technical synonym for mandíbula. It is primarily used in medical, dental, or scientific contexts.
El dentista necesita una radiografía de tu maxilar superior.
(The dentist needs an X-ray of your upper jawbone.)
La cirugía ortognática busca alinear correctamente ambos maxilares.
(Orthognathic surgery aims to correctly align both jawbones.)
Sufrió una fractura en el maxilar durante el accidente de coche.
(He suffered a fracture in his jawbone during the car accident.)
El especialista en ortodoncia evaluó el desarrollo de mis maxilares.
(The orthodontist evaluated the development of my jaws.)
En anatomía, el maxilar inferior es el único hueso móvil de la cabeza.
(In anatomy, the lower jawbone is the only mobile bone in the head.)
Summary
In short, use mandíbula for the jaw in all general conversations; it specifically refers to the lower, movable jaw. Use maxilar in technical, medical, or scientific contexts, especially when you need to be precise about whether you mean the upper jaw (maxilar superior) or the lower jaw (maxilar inferior).