Bombilla vs. foco vs. bombillo
Spanish, like many other languages, has regional variations when it comes to words. This includes the terms 'bombilla', 'foco' and 'bombillo', which all mean 'light bulb'. However, their usage can depend on the specific country or region.
Bombilla
'Bombilla' is commonly used in Spain to refer to a light bulb.
Por favor, cambia la bombilla del salón.
(Please, change the light bulb in the living room.)
La bombilla de la cocina está parpadeando.
(The kitchen light bulb is flickering.)
Foco
'Foco' is used in Mexico and some other parts of Latin America as a synonym for 'light bulb'. However, it also refers more broadly to a spotlight or focus of attention.
Necesitamos un nuevo foco para el farol de la entrada.
(We need a new light bulb for the entryway lantern.)
El foco de atención está sobre el escándalo político.
(The focus of attention is on the political scandal.)
Bombillo
'Bombillo' is also a word that refers to a light bulb, and it is mainly used in Colombia, Venezuela, and some parts of Central America.
El bombillo de la habitación no funciona.
(The light bulb in the room does not work.)
Voy a reemplazar el bombillo de mi habitacion.
(I'm going to replace the light bulb in my room.)
Summary
'Bombilla', 'foco', and 'bombillo' all refer to what we call a `light bulb` in English but their usage differs based on geographic location, `bombilla` being most common in Spain, `foco` being more dominant in Mexico's lexicon and `bombillo` appearing frequently in Colombia, Venezuela and parts of Central America. As with many aspects of language, regional variations account for these differences.