'rectilinearity' originates from Latin, specifically the words 'rectus' meaning 'straight' and 'linea' meaning 'line', formed in English from the adjective 'rectilinear' plus the noun-forming suffix '-ity'.

'rectilinearity' changed from the adjective 'rectilinear', which itself comes via Medieval Latin 'rectilinearius' (from Latin 'rectus' + 'linea'), and eventually became the modern English noun 'rectilinearity' by adding the suffix '-ity'.

Initially, it meant 'the state or quality of being straight or consisting of straight lines', and over time this basic meaning has largely remained the same in modern usage.