Acrônimo vs. acrónimo vs. sigla
The Portuguese words acrônimo, acrónimo, and sigla all refer to abbreviations formed from the initial letters of words, but they have subtle linguistic and regional differences. The primary distinction lies between whether the abbreviation is pronounced as a natural word or letter by letter, as well as a spelling variation between Brazilian and European Portuguese.
Acrônimo
In Brazilian Portuguese, an acrônimo is an abbreviation formed from the initial letters or syllables of a phrase that is pronounced as a single new word, rather than letter by letter.
O professor pediu para a turma criar um acrônimo criativo para o projeto.
(The teacher asked the class to create a creative acronym for the project.)
É comum confundir um acrônimo com uma simples abreviatura ao estudar gramática.
(It is common to confuse an acronym with a simple abbreviation when studying grammar.)
Muitas pessoas não sabem que essa palavra começou como um acrônimo militar.
(Many people do not know that this word started as a military acronym.)
Um bom acrônimo deve ser fácil de lembrar e de pronunciar.
(A good acronym should be easy to remember and pronounce.)
Acrónimo
This is the European and African Portuguese spelling of acrônimo, carrying the exact same meaning but written with an acute accent to reflect the open vowel pronunciation used in these regions.
O jornalista usou um acrónimo no título do artigo para poupar espaço.
(The journalist used an acronym in the article's title to save space.)
Em Portugal, a pronúncia da palavra acrónimo tem a vogal aberta.
(In Portugal, the pronunciation of the word acronym has an open vowel.)
O novo partido político escolheu um acrónimo que soa como uma palavra de esperança.
(The new political party chose an acronym that sounds like a word of hope.)
Tivemos que pesquisar o significado daquele acrónimo no dicionário.
(We had to search for the meaning of that acronym in the dictionary.)
Sigla
A sigla is an initialism, which is an abbreviation formed from the first letters of a multi-word name and typically pronounced letter by letter. However, in everyday Portuguese, sigla is widely used as a blanket term for all initial-based abbreviations.
Você precisa preencher o formulário usando a sigla do seu estado.
(You need to fill out the form using the initialism of your state.)
O documento oficial continha uma sigla que ninguém no escritório conhecia.
(The official document contained an initialism that no one in the office knew.)
Eles decidiram mudar a sigla da empresa após a fusão internacional.
(They decided to change the company's initialism after the international merger.)
Sempre escreva o nome completo antes de usar a sigla pela primeira vez no texto.
(Always write the full name before using the initialism for the first time in the text.)
Summary
In short, acrônimo and acrónimo mean exactly the same thing—an abbreviation pronounced as a word—differing only in whether you are using the Brazilian or European Portuguese spelling. On the other hand, a sigla strictly refers to an initialism pronounced letter by letter, although native speakers frequently use sigla as a general term for any abbreviation made of initials.