The word 'today' is a fossilized prepositional phrase, descended from Old English 'tō dæge,' meaning literally 'on (this) day' or 'to (this) day.' Over centuries of use, the two words fused into one, losing their separateness while preserving their combined meaning. It is a transparent compound: 'to' + 'day,' though the original 'tō' meant 'on' or 'at' (a temporal sense of the preposition) rather than the modern directional 'to.'
The Old English noun 'dæg' (day) descends from Proto-Germanic *dagaz, a word common to all the Germanic languages: German 'Tag,' Dutch 'dag,' Swedish 'dag,' Danish 'dag,' and Old Norse 'dagr.' The PIE etymology of *dagaz is debated, but the most widely accepted theory connects it to *dʰegʷʰ- (to burn, to be warm), making the 'day' the 'burning time' or 'warm time' — the period of heat and light, as opposed to the cold darkness of night. An alternative theory connects it to *dʰeh₁- (to set, to place), but the 'burning' etymology has more supporters.
The formation 'tō dæge' was not unique to English. German 'heute' (today) follows an exactly parallel pattern: from Old High German 'hiu tagu,' literally 'this day' (with 'hiu' meaning 'this' in the dative and 'tagu' being the dative of 'tag'). Dutch 'vandaag' (today) is from 'van den dag' (from/of the day). In each Germanic language, 'today' was originally a phrase that crystallized into a single word.
The pattern extends to 'tonight' (Old English 'tō niht,' to/on this night), 'tomorrow' (Old English 'tō morgen,' to/on the morning/morrow), and the now-obsolete 'to-year' (this year) and 'to-morrow' as two separate words. English lost the hyphenated spelling 'to-day' during the twentieth century — newspapers and publishers gradually adopted the solid form, with 'The Times' of London holding out until 1928.
The figurative use of 'today' to mean 'the present era' or 'modern times' is also old. 'The youth of today,' 'today's world,' 'today's technology' — the word extends from the literal present day to the broader present moment in history. This temporal broadening is natural for a demonstrative time-word: 'today' points to 'now,' and 'now' can mean this moment, this day, this year, this century, or this era, depending on context.
The word 'day' itself has undergone its own rich semantic development. Beyond the literal period of sunlight, 'day' means a 24-hour period (one rotation of the Earth), a working shift ('the day shift'), an era ('in my day'), and figuratively a period of power or success ('his day will come,' 'she has had her day'). The compound 'today' inherits all of these overtones.
Linguistically, 'today' illustrates one of the most common processes in language change: the fusion of a syntactic phrase into a single lexical word. Preposition + noun combinations regularly become adverbs in English: 'ashore' (on shore), 'aboard' (on board), 'afield' (in the field), 'ahead' (at the head). 'Today,' 'tonight,' and 'tomorrow' are the most successful examples of this process — phrases so frequently used that they became inseparable.