Although still in use in British English for more technical words (such as œstrogen or cæsium), the æ/œ spelling (known as a ligature, in which two or more letters are graphically combined) is now not universally used for most other words that were traditionally written with a ligature. These words now have a single written vowel that substitutes the older æ form (medieval and encyclopedia being well-known examples). Opt for the simplified spelling for all such non-technical words.