The term abbreviation has its origin in the Latin language, and it is an orthographic type procedure that supposes the reduction of a word through the suppression of final or central letters and that, in general, ends with a period. Some examples are: Atte. (for "yours sincerely"), Mr. (for "sir"), Dra. (for "doctor").
Although the aforementioned abbreviations (and many others) are in widespread use and have taken on a conventional form, anyone who knows how to write can create an abbreviation for their personal use. When a writer does this, he usually includes a glossary at the beginning or end of his work where he explains the particular abbreviations he has used.
There are two ways to abbreviate a term: by truncation or by contraction. The abbreviation by truncation consists of eliminating the final part of the word, as can be seen by observing Av. (for “avenue”), or etc. (for "etc.") In the Spanish language, this type of abbreviation never ends in a vowel, unlike in other languages, such as English (see Ave., which corresponds to the term "avenue", which means avenue).
This leads us to deal with a very common mistake made by foreign language students, or by those who consider that they know how to speak a language simply because it has certain superficial similarities with their own: transferring the orthographic rules of their native language to the foreign one. Both punctuation and word division, stress, grammar and of course abbreviations can vary substantially from one language to another, and it is essential to take this into account in order to learn correctly.
The most usual thing, regardless of the construction of the abbreviation itself, is that it is assumed that all of them have a full stop. If we take “doctor” as an example, one of the most commonly abbreviated terms in several languages, it is very likely that many people believe the correct abbreviation Dr. in any Romance language, including Spanish, Portuguese, Italian and French, among many others. In the same way, given that the Anglo-Saxon term “doctor” is widely known and that it has the same spelling as its corresponding Spanish, it is surely considered natural that it be abbreviated as in Spanish.
Using this last case in particular, if we refer to the English rules for constructing abbreviations, we will notice that if its last letter corresponds to the last letter of the original term, then it must not end with a period. This makes it clear that between the Castilian "Dr." and the English "Dr" there is a difference, as small as it may seem, and they should not be used interchangeably.
The abbreviation by contraction, on the other hand, supposes the elimination of the central letters of the word, leaving only the most representative ones; some of the most used are Avda. (for “avenue”) and no. (for “number”).
There are abbreviations that can end in flown letters, such as n.º (for “number”). On the other hand, there are abbreviations in which the final point of a word is replaced by a forward slash: c/c (for "current account") or c/u (for "each one"). There are even abbreviations that can be written in parentheses: (a) (for “alias”).
It should be noted that, according to the orthographic rule, abbreviations should keep the tilde of the word of origin, as is the case with p. (for “page”).
The acronym and the acronym are other types of abbreviations and are often confused with each other and with the abbreviations, although in essence they are clearly different: the acronym consists of taking the initial letter of each word that makes up a complex type expression, as is the case GDP (Gross Domestic Product); the acronym, on the other hand, uses more letters of the terms it combines, so that its sound is equivalent to that of a word, as is the case with MERCOSUR (Common Market of the South).