How to use the word ma'am. When and in which situations can I use this word? Which women can I greet with this word?
I have seen this word being employed in the Harry Potter and the Philosopher's Stone movie when Ron Weasley (Rupert Grint) spoke to professor Minerva McGonagall (Maggie Smith). But this is a fairy tale!
It depends on what country you're in. In Ireland (where I'm from) and the UK (where I live right now) 'ma'am' is pretty much never used. However, sometimes people working in shops/restaurants etc might use 'madam' – but it would be very strange to hear it outside that context. It's used for customers, not friends or acquaintances.
I think that in this part of the world, 'ma'am' sounds too old-fashioned.
If you are visiting the UK or Ireland and you are not an employee in a shop, you don't have to call anyone anything! If you need to ask someone for directions on the street, for example, just say 'excuse me' instead of 'excuse me ma'am/madam' etc. It's not necessary for politeness. In fact, using 'madam' (or 'sir') would most likely sound too formal for this context.
In the East Alabama speech community I grew up in, ma'am was the feminine equivalent of sir addressed to men. It was conventional to use it to all women older than the speaker, and to younger women with whom the speaker was not on familiar terms.
I myself use both ma'am and sir to everybody, including the people I work with and very young children, who seem to take special pleasure in being treated as if they were grown-ups. (The children, that is—my colleagues are happiest when they're left alone to do their work.)
FOLLOWUP:
The consensus appears to be: if you're in the American South, use ma'am generously to any woman, pronouncing it /mæm/. Elsewhere, save it for a) your female superiors in the military and b) the Queen on your second and subsequent remarks to her.
For me, this can be a loaded question.
Ma'am is often used in the military instead of "Sir", to a female of higher rank.
Ma'am is used to note respect to a woman with a noble title. You would not always say "Lady Susan", the maid could answer, "Yes, Ma'am." (In England, it often sounds like "Mum".)
Teachers in some institutions are called ma'am at the same times a man would be addressed as "sir". (We used "Miss" in my school.)
In the Southern US, a woman is called ma'am as a sign of respect. So a store clerk might say, "Here is your package, ma'am." In other places in North America, ma'am is a sign of respect.
This is why it can be loaded. I am Canadian and in the part of Canada I am from, a woman might easily feel like you are saying she is old. So a store clerk would call most women "Miss" instead. This doesn't mean they are unmarried, just that the woman is not old.
In the context of English schools:
Prior to the second half of the 20th century, male schoolteachers would be referred to as "Sir" and female schoolteachers as "Miss". At that time, a married woman would not normally be permitted to remain as a teacher, so teaching was a career for unmarried women. With changing social attitudes women were not compelled to resign after marriage. This caused the conventions on what to call teachers to change.
Some schools continue to use "Miss", seeing this a word of respect, not an indication of marriage. Some use "Mrs Smith" (and Mr Smith for men), preferring to use names. Some shifted to "Ma'am" to refer to female teachers, based on how one would refer women of higher rank (in the army, for example). This is part of the culture of the school.
In general, outside of school, avoid "Ma'am" (and Sir). In business (for example) you should get to know people's names, and then use their names. It is reasonable to ask for someone's name if you are in a business meeting. In casual meetings (asking directions for example) you would not normally need names, or titles. If that is impossible to avoid (for example as a shop assistant speaking to a customer) you may use "madam" and "sir". And if you meet the queen, use "your majesty" at first, and "ma'am" on subsequent occasions.
@StoneyB and @Willow Rex have offered some useful observations.
I would like to add a few other points:
In my experience, in the US it is still generally considered polite, and largely obligatory, whether in the South or outside the South, for anyone representing a business establishment or a government office to address a customer or member of the public whose actual name the worker does not know as "sir" (for men) or as "ma'am" (for women). This applies to servers in restaurants, desk attendants in hotels, flight attendants and other airline staff, company customer service representatives answering telephone calls, police officers, clerks in government agencies, etc.
This also applies to interactions between adults who do not know each when one is trying to attract the attention of another in a public space, such as while riding on a bus, while waiting in a queue when shopping or when entering a business, etc.).
It is considered extremely rude to try to get someone's attention by saying something like "Hey, you," or "Hey, lady," rather than "Excuse me, ma'am."
("Madam" is very rarely used in spoken English. It is generally considered extremely formal and old-fashioned.)
Younger women and girls are still usually addressed in these situations as "miss" rather than "ma'am," while older women are addressed as "ma'am." There is no absolute rule for when a woman is too old for "miss" and should be addressed instead as "ma'am." (I was taught years ago that you should use "ma'am" for any woman who is old enough to be a mother. lol)
As a counterpoint to the other examples: While there are many situations and locations where the use of "ma'am" is common and appropriate, having lived in California for most of my life I can't recall even one instance where I used it as it is meant to be used. It can be said by those in service or retail jobs where they are expected to show politeness and deference to the customer -- the more "upscale" the business, the more likely they will use "sir" and "ma'am".
But it is not normal in casual conversation, so much so that when I met a young man who punctuated every sentence with "sir" or "ma'am", his good manners seemed extraordinarily and almost uncomfortably archaic.
Which isn't to say that young people here are rude and don't use any honorifics. Instead it's more common to call someone my their name, "Mr. Jones", "Mrs. Smith", etc.
Anyway, I'm now at an age where I am (from time to time) called "sir" -- which, as others have pointed out, makes me feel old. So I would never call someone else "ma'am", lest they feel that I was implying they are even older than I am.
