'intergalactic' is formed from the Latin prefix 'inter-' and the adjective 'galactic' (from Greek), where 'inter-' meant 'between' and the Greek root 'galakt-' (via 'galaxias') referred to 'milk' (hence 'galaxy' = 'milky [way]').

'galaxy' comes from Greek 'galaxias' (related to 'galakt-,' meaning 'milk'), passed into Latin and then Middle English as 'galaxie'/'galaxy'; the adjective 'galactic' was formed from 'galaxy' + '-ic', and 'intergalactic' was created by prefixing Latin 'inter-' to 'galactic' in modern English usage to mean 'between galaxies.'

Initially the components referred literally to 'between' and 'milk/galaxy'; the combined modern word has meant 'between galaxies' since formation, and this core sense has largely been retained, with extended figurative uses (e.g., in science fiction).