Signora. The short form is Sig.ra. In Italian there is no difference between married and not married. It is always signora. Although signorina refers to an unmarried woman, it is considered somehow informal no matter what.

Keep in mind that if she has a degree, and you'll know by the business card or her letterhead, you'll have to go with the 'Dr.ssa (dottoressa)", or whatever specific title she might have.

The word "signorina" is still in use: even though if you don't know the woman you'll probably address her as "signora" some people nowdays will still call a very young woman "signorina", since they assume that a girl of 18 o 20 years is not married yet. On the contrary, even though a woman of 50 or more may still be unmarried, we find somewhat ridiculus to address an old woman this way unless of course we know her very well. Sometimes I hear people ask, when they are about to address a woman "signora o signorina?" if they want to be sure to avoid any blunder. Of course, there may be women who don't like this kind of distinguish, but definitely the word is still used, although less than in the past. Here's what say the "Treccani dictonary" signorina s. f. [dim. di signora]. – 1. Titolo di cortesia e di rispetto con cui ci si rivolge o ci si riferisce a donna giovane non sposata, premesso al cognome (la s. Rossi) o con più familiarità, al nome (la s. Claudia). Anche assol.: buon giorno, signorina; si accomodi, signorina; spesso scherz., parlando a bambine: vogliamo smettere, signorina, di fare i capricci? Ha in genere usi analoghi a quelli di signora, come, per es., in indirizzi di lettere o di altre missive (in tal caso per lo più nell’abbreviazione sig.na): Alla sig.na Claudia Bianchi. Scherz. s. buonasera, annunciatrice televisiva.

the term "signorina" was abolisched 20 years ago with law... femminist reason...