I have come across at least 2 words: pila and fregadero. Do they really mean exactly the same thing or are there any subtle differences? Can they also be used for the washbowl that is not in the kitchen? Are there any other words in Spanish for the sink or washbowl or wash-basin?
Both are ok, it depends on where are located at your house.
Pila is according to RAE:
Pieza grande de piedra o de otra materia, cóncava y profunda, donde cae o se echa el agua para varios usos.
And fregadero is a pila where to wash/clean. According to RAE:
Pila de fregar.
In Spain fregadero can be outdoors at a terrace for example when you are speaking about the one you use to clean your clothers so you will hang the clothes out to dry, or the fregadero inside the kitchen used to wash/clean your plates.
Pila is in the bathroom.
About washbowl is palangana
In Mexico, equivalents of the word sink (meaning the bowl where the dishes are washed) are tarja and fregadero. There is no difference whether it is indoors or outdoors; it usually has one or two bowls and made of metal or porcelain.
Lavadero is a stone made piece used to wash clothes (Mexico.) Usually a lavadero has a pila attached.
Pila/pileta is an uncovered stone piece made with the purpose of storing water; usually to wash something but this is not a must (its use meaning sink is also regional.) Another meaning for pila/pileta is pool (also regional.)
Lavabo/lavamanos is usually in the bathroom or just outside the bathroom and used to wash ones hands. It is commonly made of porcelain, but other materials can also used.
Palangana can have different regional meanings but in Mexico is a portable water container (i.e. is not attached to buildings.) It can be made of plastic or metal.
The water storage for buildings is called aljibe/cisterna if it is underground; tinaco if it is elevated or placed on the rooftop. These are usually covered because they store clean water.
fregadero is the one you have in your kitchen and you use to wash your dishes. Although
pila can be understood as a synonym of
fregadero, Spaniards would favor the latter.
Qué quieres que haga con estos platos?
Déjalos en la pila / Déjalos en el fregadero
"Pila" is a concave and deep piece of stone (or steel or other material) where you put water for different uses. You could find a "pila" in a church (known as pila bautismal). Also, note that when you are asking somebody to "dejar los platos en la pila" it could also have the double meaning of "leave them in the stack", if you are putting plates on top of each other in order to be washed.
Spaniards tend to use the word
lavabo to refer to the sink in the bathroom (where you would wash your hands). You could also hear "la pila del baño", to refer to "lavabo", but again most people would favor "lavabo" over "pila". "lavabo" is also how the bathroom can be know when it doesn't have a shower or bath (has just the sink and a toilet). Most bathrooms in Spain have a
bidé (to wash other things). A bidé could be understood as another pila, but it receives that name (etymology from the French bidet, the name of a breed of small horses).