Origins
The English word 'pillar' entered the language in the thirteenth century from Old French 'pilier' (modern French 'pilier'), which descended from Vulgar Latin *pilāre, a derivative of Latin 'pīla.' The Latin word 'pīla' had two primary meanings: a pillar or pier, and a ball or sphere. Whether these represent one word with divergent meanings or two distinct homophones has been debated for centuries, but the architectural sense — a thick vertical support — is clearly the ancestor of English 'pillar.'
Latin 'pīla' in its architectural sense referred to a solid, freestanding support structure, particularly the massive piers of bridges and aqueducts. Roman engineers built pīlae as the foundations of their greatest hydraulic works: the piers of the Pont du Gard, the bridge supports spanning the Rhine, and the breakwaters (moles) of harbors. The word thus carried associations of massiveness and engineering solidity.
Some etymologists trace 'pīla' to the PIE root *peyH-, meaning 'to be fat' or 'to swell,' which would make a pillar literally a 'swollen mass' — an apt description of the thick, drumlike stone cylinders of Roman architecture. The same root may underlie English 'pile' (a heap), which entered from Latin 'pīla' by a slightly different path.
Latin Roots
The relationship between 'pillar,' 'pile,' 'pier,' and 'pilaster' is complex. 'Pier' entered English from Medieval Latin 'pera,' possibly a variant of 'pīla.' 'Pilaster' (a rectangular column projecting from a wall) came from Italian 'pilastro,' from Latin 'pīlastra,' a derivative of 'pīla.' All three words share the same Latin ancestor but arrived in English through different routes and at different times, each carrying a slightly different architectural meaning.
In its figurative sense, 'pillar' has been used since the fourteenth century to mean a person who provides essential support or strength — a 'pillar of the community,' a 'pillar of strength.' This metaphor draws on the architectural function of a pillar as something that holds up a structure that would collapse without it. The biblical 'pillar of salt' (Lot's wife in Genesis) and 'pillar of cloud' (guiding the Israelites) gave the word additional figurative weight in English.
The phrase 'from pillar to post' (driven from one place to another without rest) dates from the fifteenth century. Its exact origin is disputed: some trace it to the pillars and posts of a tennis court, others to the pillory (pillar) and whipping post of public punishment. The expression captures a sense of being knocked between two immovable vertical structures.
Later History
The 'pillar box' — the distinctive red freestanding postbox of the British postal system — dates from the 1850s and takes its name from its cylindrical, pillar-like shape. These were introduced under Anthony Trollope, better known as a novelist, who served as a Post Office surveyor and recommended the design based on models he had seen in France and Belgium.
In architectural terminology, there is a distinction between a pillar (any vertical support, of any cross-section) and a column (a vertical support with a specifically circular cross-section, usually with classical proportions including a base, shaft, and capital). In popular English, however, 'pillar' and 'column' are used nearly interchangeably, with 'column' slightly more common in technical architectural writing and 'pillar' more common in figurative and everyday usage.
German 'Pfeiler' (pillar, pier) was borrowed from the same Latin source through a different Old French intermediary, illustrating how a single Latin word could enter the Germanic languages through multiple channels during the medieval period.