The formula 'Señora de' or 'Viuda de' is almost unused nowadays. You won't find anyone younger than 60 who call themselves that way. And, as Fernando says, it is not a legal name, simply an old fashioned way to be known.

About the colín, it is the name of a stick of bread, that can be short (about one inch, like those in Andalucia, known as picos, too) or really long (up to two feet, like those sold in Madrid, unfortunately in less and less bakeries).

Thank you everyone for the clarifications. Oh yes, the 'Señora de' or 'Viuda de' died out a long time ago; even amongst Latin Americans.

My experience is that for non-Spaniards, the Spanish government and most businesses (including banks) just take your name as it is recorded in your passport.

I had a Spanish I.D. card for U.S. military members stationed in Spain. I also had a Spanish driver's lic. They just used my normal name - no one made a big deal of it.

Maybe someone can help me? I am planning to write a letter to my friend in Spain who lives with his sister in her flat.How would I address the envelope? In the States I would address as follows:
John Smith
c/o Jane Doe
What would the correct way be in Spain?
Thanks!

A la atención de: Mr. John Smith

Jane Doe
C/ (name of street), (number of the street)
(postal code) - (city)
SPAIN

Gracias Fernando!

You're welcome