When to use scorsa and ultima

Ciao:

Is it correct that "scorsa" and "ultima" both translate as "last"? If correct, are these two words interchangeable?

Thanks
AR

Ciao AR, 

The words "scorso" (or scorsa - feminine) and "ultimo" (or ultima - feminine) translate as "last", that's correct. 

Sometimes they can be interchangeable depending on the context and the word they refer to, but there is actually a slight difference.

"ultimo" usually refers to the "last one of many", while "scorso" simply refers to the "last one".

If you want to say "Last week I went to Rome" you can say "La scorsa settimana sono andato a Roma".  If you say "L'ultima settimana sono andato a Roma" it might imply that you went to Rome on the last week "of many" (perhaps of many weeks of holiday).

However, the sentence "L'ultima mostra era interessante" (The last exhibition was interesting) and "La scorsa mostra era interessante" refer to the last event taking place (but the word "ultima" again implies that it was the last one of a series of exhibitions).

In certain contexts they are not interchangeable:
Last night I went out = La notte scorsa sono uscito (i.e. yesterday night I went out)
If you say "L'ultima notte sono uscito" you are referring to the last night of a series of nights. 

Let me know if you have a specific sentence in mind or a specific context, so that I can make it even clearer for you using it as an example :)

Thanks!

Ciao Caterina,
Grazie per la spiegazione.

AR