Is the word "stool" an informal word or a formal word?

Is the word "stool" an informal word or a formal word? I think it is a kind of formal word, especially a medical word. It is used in several academic papers and articles, and also doctors like using this word when they want to talk about status of patients' excrements. However my English teacher(not native speaker) said that the word stool is an informal word, like poo and I should use "feces or excrement" as formal language. So I told him that I think it looks like a formal word than informal word because basically it is a specialized word. Then he said that "Native speakers" use the word stool in informal conversation as using phasal verbs like"have a stool", "after stool", or "go to stool". I have never heard of those kind of expressions before. Is he correct?

It is only ever used in a formal medical sense, with examples from the sixteenth century.

Stool derives from the name given to an enclosed chamber, or commode, used for producing stools.

The most usual form is in the plural.

d. A discharge of fæcal matter of a specified colour, consistency, etc.; the matter discharged (chiefly pl.). (OED sense 5d).

It is certainly NOT a slang expression, and I have no idea why it would appear in street dictionaries.

"Stool" is more formal than poop or poo but sometimes more comfortable to say than feces or excrement.

You could say "I've been having loose stool" to express that you don't quite have diarrhea, but it's somewhat in that direction.

An example from Angel in Disguise: A Memoir

The next morning after the children had their breakfast, I asked Verna “Have you fed the baby?”

“She's not hungry', she said.

“Does she still have a loose stool?” I asked.

“No more”, she said. “She just wants to sleep”.

You could also say "I have to give a stool sample" or "I have to get a stool sample from my dog to bring to the vet".

You could also use the word stool if you need to describe your or your baby's poop, such as "there's corn in my stool", "my baby has tarry stool", "bloody stool", "black stool", "hard stool", "watery stool", etc.

Or "Mom, I swallowed some sand"; "Don't worry, it'll come out in your stool".

And how could I forget, "Honey, could you pick me up some stool softener when you go to the store?"

So stool isn't so formal that no one uses it in conversation, but it's use is limited to a few particular situations.

Be interesting to hear other examples of what the teacher considers to be formal words.

How did the teacher define 'formal'? Does it mean words only used in certain settings or instances, words not commonly in use or just words that are not usually thought of, or perhaps are not specifically defined, as colloquialisms ?

English is widely spoken around the world but there are distinct differences in usage depending where you learned to speak it and there are numbers of distinct dialects as well, so I wonder if that's part of the problem. I know I had teachers who were, respectively, Australian, Egyptian and East Indian and they all used certain terms and words that were unfamiliar.

I'd expect average Australian, UK, US or Canadian English speakers should be able to understand one other without much difficulty, but there would surely be plenty of unfamiliar terms or usages that differ.

I'm a Canadian born English speaker with a very wide vocabulary. I've been known to read dictionaries just for fun. I often get puzzled looks over some of the words I use, because a good many of them simply are not part of the distressingly small common vocabulary one usually hears in everyday conversation.

I can only tell you that the word stool, as it relates to bodily waste, is not a word I've ever heard used by anyone in ordinary conversation. Diarrhea, constipation and various euphemisms for various digestive upsets or conditions are very common to hear, but not the word stool. If I hear it at all, it's in reference to a medical topic. Possibly something like, 'The vet asked me to remember to bring a stool sample when I take the cat in for her checkup'.

I think the only time I've ever heard the word used as part of anything you might consider as ordinary conversation, of a sort, was on a TV sitcom, not too long ago. This was during an episode of a show called The Big Bang Theory. It was used by the character called Sheldon, referring to bodily waste. The 'Sheldon' character speaks in a stilted, highly formal way and is depicted as a highly educated genius who is incapable of reading body language or emotional situations between people and is seriously inept with all social interactions.

So truly, the only common use I know for the word is where it refers to a backless seat.

The only times I would expect to hear the word stool used would be by a doctor, a vet, or other medical personnel or in reference to something associated with a medical topic. Most usually it's used with the word sample, as in 'can you provide a stool sample'.

I agree with the other posters that the other examples given by the teacher, 'go to stool' or 'after stool' are nothing I've ever heard and would strike me as being very awkward usage at the very least.

Around here, the phrase 'take a crap ' or sometimes, 'have a dump' indicate that someone needs to move their bowels.

Although, as others have noted, stool is mainly used when discussing medical conditions related to fecal matter and is almost never used informally, it does appear in other contexts once in a while.

For example, in a classic Saturday Night Live skit, Phil Hartman, playing Frank Sinatra, says to Sting, playing Billy Idol,

"You don't scare me. I've got chunks of guys like you in my stool!"

(Video of quote here; video of entire skit here (stool quote is near the end); transcript of skit). The use of stool, rather than one of the many informal or vulgar words for fecal matter one might expect in this context, likely contributed to the humor.