Loanwords
What English took from Arabic, what modern Arabic has taken from English, and the long shadow Arabic cast over Spanish.
Loanwords are not trivia. They are evidence — of trade routes, of conquests, of which culture had the prestige in a given field at a given time. The Arabic words that entered English in the medieval period cluster heavily in mathematics, astronomy, chemistry, and navigation: algebra, algorithm, zenith, nadir, alcohol, cipher, admiral. The English words now appearing in Arabic, by contrast, cluster in technology, business, and consumer culture: kompyuutar, internet, mobaayl, marketing. Each direction tells a different story about its century.
Read the pages in whatever order suits you. Arabic words in English is the most-asked-about and probably the page to start with: it is much longer than most English speakers expect. English words in modern Arabic covers the live, ongoing borrowings, including the ones that purist editors try to suppress and the ones that have already settled in. Arabic-origin words in Spanish is a side trip but a rewarding one — roughly eight percent of the Spanish vocabulary, by some counts, traces to Arabic, the legacy of nearly eight centuries of Muslim presence on the Iberian peninsula. False friends catalogues the words that look or sound like cognates but are not, or that are cognates with shifted meanings. Complete etymology list is a longer reference, organized alphabetically.
One discipline we apply to this section: we cite a word as Arabic-derived only when the standard etymological dictionaries agree on the path. Folk etymologies are everywhere on the internet — checkmate from الشيخ مات "the king is dead" is the famous example, more often repeated than verified — and we try not to add to them. Where the path is contested, we say so.