Medieval Portuguese developed between the 12th and 15th centuries from Galician-Portuguese. This period is marked by the consolidation of a distinct linguistic system in the western Iberian Peninsula. The formation of the Kingdom of Portugal in 1139 favored the stabilization of written practices distinct from Latin and from other Iberian varieties. Documents such as the Testament of D. Afonso II illustrate the transition from Latin to written Portuguese. Contact with other languages continued to influence the lexicon and the structure of the language.

The lexicon of medieval Portuguese is organized into main layers of origin: Vulgar Latin, pre-Roman substrates, and Germanic contributions. These layers form the core of the everyday vocabulary and the central terms of the grammatical structure. Throughout the Middle Ages, the lexicon also received influences from Arabic, French, and Occitan, especially in cultural and technical areas. The differentiation of registers and the formation of neologisms occurred within this framework of stable bases and selective loans.

Vulgar Latin gave rise to the majority of the basic words of medieval Portuguese through phonetic and morphological evolution. This heritage includes everyday terms, pronouns, numerals, and central verbal structures. The transition from Latin to Galician-Portuguese involved regular sound changes, reduction of cases, and reorganization of the verbal system. Many of these forms remain recognizable in modern Portuguese. Classical Latin continued to exist as a language of written prestige, but with a different circulation from the inherited vulgar Latin.

Substrates are words and traces preserved from languages predating Latin in the Galician-Lusitanian region. In the lexicon, they appear mainly in names of natural elements, animals and geographical features. These terms tend to be old in the oral tradition and not closely tied to Roman administration. In medieval Portuguese, substrates coexist with Latin forms without systematic conflict. Their identification depends on historical reconstruction and comparison with other areas of Western Europe.

The Germanic influence on medieval Portuguese arises primarily from the presence of the Suebi and Visigoths. The loans concentrate in fields such as war, administration, society, and to a lesser extent, toponyms. Many of these words entered Galician-Portuguese already integrated into the local vulgar Latin. The stability of Germanic terms depended on their usefulness in the new social systems of the High Middle Ages. Some forms underwent phonetic adaptations compatible with Romance patterns.

Arabic left marks on medieval Portuguese through contact with Al-Andalus and cross-border coexistence. The loans are especially frequent in agriculture, irrigation, instruments, commerce, and scientific terminology. Many of these words arrived with associated technological or administrative innovations. The adaptation to Galician-Portuguese followed local phonetic rules, creating stable forms in medieval texts. Several of these terms retain clear identification in modern Portuguese.

French and Occitan influenced medieval Portuguese culturally, especially through troubadours, chivalry, and the church. These loans appear frequently in vocabulary related to literature, music, law, and urbanism. They often entered through contact with cultural centers and the circulation of manuscripts. In Galician-Portuguese, these terms could coexist with Latin-base equivalents or exist as higher-register words. Fixation depended on the prestige of the associated cultural models.

The evolution from Latin to medieval Portuguese involved regular phonetic changes that created new and distinctive forms. Among the most important are palatalization, diphthongization, and the reduction of unstressed vowels. These changes explain systematic differences between Latin words and their medieval descendants. In the medieval period, these rules were already widely established in texts. Medieval writing reflects these evolutions with some graphical variation, but with a consistent phonological basis.

Medieval Portuguese used derivatives through suffixation and prefixation with Latin bases and local adaptations. Suffixes such as -eiro, -ança, and -mento were productive in forming nouns and agents. Composition also occurred, especially in technical and toponymic terms. Many medieval derivatives stabilized through frequent use in documents and literature. Morphological transparency facilitated vocabulary expansion without breaking with the inherited structure.

In medieval Portuguese, different registers coexisted, with Latin as the formal language and Galician-Portuguese as the language of royal documentation and lyric poetry. Writing showed graphic variation due to the lack of strict standardization. Many written forms approached local pronunciation and could vary between scriptorium and region. The fixation of formulas and legal terms helped stabilize parts of the vocabulary in documents. The transition to classical Portuguese increased the pressure for written standardization, without eliminating the medieval base.

Medieval Portuguese formed from Vulgar Latin, integrated with substrates and with Germanic, Arabic, and neighboring Romance loans. Its words evolved through regular phonetic changes and productive morphological formation. The resulting lexicon appears in medieval texts with graphic variation and strong continuity with modern Portuguese. Understanding these roots explains why many current terms retain traces of the medieval period. Medieval etymology is a key to interpreting the internal history of Portuguese.