What is a colloquial/slang expression for a cigarette in Brazilian Portuguese?
I only know that the formal one is "cigarro m".
Thank you.
"Cigarro" is both formal and colloquial.
Some slang words and phrases are:
"pito", "crivo", "de branco" ([dressed] in white) - any cigarette
"mata-rato" (rat poison), "estoura-peito" (breast burster) - cheap, strong, bad cigarette
"guimba", "bituca", "bagana", "ponta" - a cigarette butt (or a joint).
Edited - also, "as vinte" (the twenty) - a cigarette butt.
(Edited because a cigarette butt is not a joint.)
ETA. Those words and expression are slang. And a good rule of thumb for any non-native speaker of any language is, "do not use slang"; the context for its use is complex and it is easy to make a fool of oneself by trying to use slang in a foreign language - even if you are quite fluent in its standard version.
"cigarro" is the most common way to refer to cigarettes in pt=BR, in both formal and informal environments. There are several other names but none of them are widely used throughout the country. Some are slang words restricted to a specific group, some are regional usage. The dictionaries mention quite a few of them but, to be honest, I never heard them.
p.s. I've heard, however, the words "guimba" (a joint) and "mata-rato" (very low-quality cigarette), as mentioned by Luis Henrique in his answer.
In addition to the other answers, one may also use the word "pigas (m)" (despite the "s", this is the singular form), as in
Me vê um pigas? (May I have a cigarette?)
I believe this is specific to the portuguese as spoken in the city of São Paulo.
Also, my friends and I sometimes use the word "cigarette", but with a Portuguese pronunciation, something like "cigaréti", but I've never heard this outside this specific group of friends.
Both words, "pigas" and "cigarette", refer to tobacco cigarettes.