Sillón vs. butaca
In Spanish, both 'sillón' and 'butaca' can be used to refer to the English word 'armchair'. However, there are subtle differences between the two that can help specify the type of seating furniture one is referring to.
Sillón
An armchair or a lounge chair that is typically large and comfortable. Sillón is often used to reference a piece of furniture designed for relaxed sitting or lounging.
Mi sillón favorito está cerca de la ventana.
(My favorite armchair is near the window.)
El sillón es tan grande que podrías dormir en él.
(The armchair is so big that you could sleep in it.)
Butaca
An armchair usually smaller than a sillón with an upright backrest. Butaca can particularly refer to seating in theaters, cinemas, or other similar establishments.
Compramos dos butacas para el salón.
(We bought two armchairs for the living room.)
Tengo una butaca reservada en la primera fila del teatro.
(I have an armchair reserved in the first row of the theater.)
Summary
While both 'sillón' and 'butaca' imply a type of chair with armrests popularly known as an 'armchair', they each have unique usages. A 'sillón' usually refers to a large and comfortable lounge chair ideal for relaxation, whereas a 'butaca' often points towards smaller chairs with upright backs, especially ones found in theaters or cinemas.