You might call it a stifled laugh , as in, “Mary tried to contain herself, but couldn’t help letting out a stifled laugh.”
You could also use the word snicker (either as a verb or noun) or possibly titter - though the latter might sound a little dated and silly.
ell.stackexchange.com/users/89131/Mixolydian
This "pfft" could be called a "snicker".
Also, sometimes the stifled laugh comes out not as a "pfft" but more of a "snort" if you try to keep the laugh inside by shutting your mouth.
At deviantart.com , you can see the entirety of a cartoon titled "What are you Laughing at?"
It may be hard to read because the lettering is so small, but the characters are trying not to laugh, and are emitting the words "snicker" & "snort" (highlighted in yellow) instead.
ell.stackexchange.com/users/40154/Lorel C.
How about the word snort .
It could be used in a sentence like: When Nick told his wife about the narrow escape from a traffic ticket
Betty tried to withhold a chuckle that came out as a snort.
ell.stackexchange.com/users/88091/Michael Karas
How about Titter?
I came across it a while ago doing a crossword on a plane.
titter /ˈtɪtə/
verb verb: titter ;
3rd person present: titters ; past tense:
tittered ; past participle:
tittered ; gerund or present participle:
tittering give a short, half-suppressed laugh; giggle. "
her stutter caused the children to titter" synonyms : giggle, snigger, snicker, tee-hee, give a half-suppressed laugh, chuckle; noun noun: titter ; plural noun: titters a short, half-suppressed laugh.
"there were titters from the gallery" synonyms : giggle, snigger, snicker, tee-hee, half-suppressed laugh, chuckle;
ell.stackexchange.com/users/96175/Hani Umer
According to wordhippo , a suppressed laugh is: A half-suppressed, typically scornful laugh
The site also offers these synonyms: snigger, snicker, giggle, chortle, sneer, cackle, chuckle, guffaw, simper
ell.stackexchange.com/users/96306/Michael Brandeis
I tend to think of guffaw, although Merriam-Webster says this is "loud laughter," how about a stifled guffaw?
ell.stackexchange.com/users/96381/L O
I think at this point pfft is an onomatopoeia.
It's a part of my vocabulary at least, but it's more like a stand in for the word pashaw .
Which is an exclamation about the hilarity of an incredulous situation.
It's a way to 'call bullshit' without swearing, or to preserve the lightheartedness of a conversation.
He let out a pfft
but I wasn't sure if he was actually annoyed, or just trying to not hurt my feelings by laughing it off.
ell.stackexchange.com/users/5117/Mazura
What do you call the small burst of laugh that people let out when they want to refrain from laughing, but can't? You know the "pfft" sound? I am not sure if it only happens when you refrain from laughing, but yeah some people I guess laugh like that.
Word for a small burst of laughter that can't be held back
ell.stackexchange.com
<word-request>
ell.stackexchange.com/questions/212723
ell.stackexchange.com/users/93377/blackbird
This "pfft" could be called a "snicker".  You could say titter..  A short, half-suppressed laugh The following are all possibilities: "stifled laugh", "snicker" "snort", "titter" and "guffaw". 
The following are all possibilities: "stifled laugh", "snicker" "snort", "titter" and "guffaw". 