This paper highlights three theoretical and descriptive insights into synonymy and lexical variation and change: (1) the diachronic development of synonymous forms reveals essential aspects about the nature and motivations of synonymy; (2) the emergence and competition of synonymous forms can either result from conceptual salience factors or from social salience factors; (3) synonym competition sheds light upon processes of language variation and change. Focusing on the interplay between conceptual and social aspects of language variation and change, this study subscribes to the framework of Cognitive Sociolinguistics, an emerging extension of Cognitive Linguistics as a meaning-oriented and usage-based approach to language. Two corpus-based case studies in lexical synonymics of Portuguese will be presented. The first case study shows the semantic development of the verb deixar 'to leave, to let' from Old to Modern Portuguese and its most competitive synonyms, namely abandonar 'to abandon' and permitir 'to allow'. The second case study includes the development of four dozens of sets of denotational synonymous nouns selected from the lexical fields of football and clothing in European and Brazilian Portuguese in the last 60 years. The two diachronic studies show essential aspects of synonymy and lexical variation and change.