Diacritics: Accents and Other Markings
These are properly known in English as diacritics, or sometimes as diacritical markings. They indicate various things, but stress is only one possibility. They also change the pronunciation of letters in other ways.
The OED says of diacritic:
diacritic /daɪəˈkrɪtɪk/, a. and sb.
Etymology: ad. Gr. διακριτικός, that separates or distinguishes, f. διακρίνειν to separate. In mod.Fr. diacritique.
A. adj. Serving to distinguish, distinctive; spec. in Gram. applied to signs or marks used to distinguish different sounds or values of the same letter or character; e.g. è, é, ê, ë, e, ē, ĕ, ȩ, etc.
B. sb. Gram. A diacritic sign or mark.
Serving to distinguish, distinctive; spec. in Gram. applied to signs or marks used to distinguish different sounds or values of the same letter or character; e.g. è, é, ê, ë, e, ē, ĕ, ȩ, etc.
Whereas the Wikipedia article on Diacritics reads:
A diacritic /daɪ.əˈkrɪtɪk/ – also diacritical mark, diacritical point, or diacritical sign – is a glyph added to a letter, or basic glyph. The term derives from the Greek διακριτικός (diakritikós, "distinguishing"), which is composed of the ancient Greek διά (diá, through) and κρίνω (krínein or kríno, to separate). Diacritic is primarily an adjective, though sometimes used as a noun, whereas diacritical is only ever an adjective. Some diacritical marks, such as the acute ( ´ ) and grave ( ` ), are often called accents. Diacritical marks may appear above or below a letter, or in some other position such as within the letter or between two letters.
The main use of diacritical marks in the Latin script is to change the sound-values of the letters to which they are added. Examples from English are the diaereses in naïve and Noël, which show that the vowel with the diaeresis mark is pronounced separately from the preceding vowel; the acute and grave accents, which can indicate that a final vowel is to be pronounced, as in saké and poetic breathèd; and the cedilla under the "c" in the borrowed French word façade, which shows it is pronounced /s/ rather than /k/. In other Latin alphabets, they may distinguish between homonyms, such as the French là ("there") versus la ("the"), which are both pronounced /la/. In Gaelic type, a dot over a consonant indicates lenition of the consonant in question.
Both Spanish and Portuguese also use diacritics to distinguish what would otherwise be homographs, usually of one syllable.
From the Wikipedia article on Portuguese Orthography we learn that present-day
Portuguese makes use of five diacritics: the cedilla (ç), acute accent (á, é, í, ó, ú), circumflex accent (â, ê, ô), tilde (ã, õ), and grave accent (à).
These are called acentos gráficos e diacríticos in Portuguese, and correspond to the cedilha, acento agudo, acento circunflexo, til, and acento grave.
Portuguese uses the tilde and the acute and circumflex accents to indicate both stress and (usually) vowel quality. It uses the grave accent for a special contraction case, and it uses the cedilla and formerly the diaeresis to indicate a non–stress-related sound change.
In contrast, Spanish uses the acute accent for stress alone not for vowel quality, while French uses the acute, grave, and circumflex accents for vowel quality but not stress.
Before recent spelling accords, Portuguese also used the diaeresis in words like lingüística, as the Spanish still do. Note that that has one acute accent mark used to indicate the stress and one diaeresis — which is not an accent mark. It is, however, a diacritic. The diaeresis is called a trema in Portuguese and various other languages.
Diacritics are most often used in loanwords in English, or sometimes for poetry. The OED has these interestingly decorated terms in it:
Allerød fête Niçoise smørrebrød
après-ski feuilleté piñon soirée
Bokmål flügelhorn plaçage tapénade
brassière Gödelian prêt-à-porter vicuña
caña jalapeño Provençal vis-à-vis
crème Madrileño quinceañera Zuñi
crêpe Möbius Ragnarök α-ketoisovaleric acid
désoeuvrement Mohorovičić discontinuity résumé (α-)lipoic acid
Fabergé moiré Schrödinger (β-)nornicotine
façade naïve Shijō ψ-ionone
Please see the Bringhurst citation in this answer regarding Unicode, ASCII, and cultural narrowness.