There were at least four towns in Western Europe that were called Mediolanum in Antiquity. The most important of these is the Mediolanum in northwestern Italy that is now known as Milano in Italian, i.e. Milan. Other towns called Mediolanum were: Mediolanum Santonum (“Mediolanum of the Santones”), which is modern Saintes in western France; Mediolanum Aulercorum (“Mediolanum of the Aulerci”), which is modern Evreux in northern France; and a Mediolanum in what is now England on the site of Whitchurch in Shropshire.

The origin of the name Mediolanum is undoubtedly Celtic (all four towns called Mediolanum were in Celtic territories that were conquered by the Romans). The Celtic form of the name was most certainly Mediolanon but beyond this the etymology is considered uncertain. Two possible etymologies that are sometimes given are:

• *medjo– “middle” + *lanon “enclosure” – the second element being considered a cognate of Welsh llan, which can mean “yard, churchyard, enclosure”. The big problem with this is that the n was originally double, as can be seen in the plural llannau, as well as the Breton cognate lann (plural lannoù).  The Welsh llan and Breton lann actually come from *landā which is cognate with Germanic *landą “land”. That being said, I consider the possibility of a Celtic *lanon meaning “enclosure” quite doubtful.

• *medjo– “middle” + *lānon < *φlānon “plain” – the second element in this case being considered a cognate of Latin plānum. Although it is certainly not impossible that Celtic had an exact cognate of the Latin word with the same meaning, I consider it unlikely that Celtic had a word for “plain” which was virtually identical to their word for “full” – *φlānos (cf. Welsh llawn, Breton leun). I suppose that one might imagine that Mediolanum meant “Middle Full(ness)”, but I’m really not keen on that possibility myself. [Added on August 18, 2022] It has recently occurred to me that the decisive argument against relating *-lānon with Latin plānus is the fact that the cognate of Latin plānus (< *plh₂t-nos) is actually Proto-Celtic *φlitanos (< *pḷth₂-nos). See the entry for platno with etymological information in my Indo-European conlang called Woks Teuteka (https://vellaunos.ca/2021/08/02/woks-teuteko/).

The etymology for Mediolanon that I personally fancy is this:

• *medjolo- “middle, centre” + *-ānon – the first element in this etymology being a noun *medjolon derived from *medjos “mid-” which is cognate with Germanic *midilą (< *midja– + *-la-) “middle”. The second element is the neuter of an adjectival ending *-ānos making *medjolānos “central”. The name Mediolanon would quite simply mean “(The) Central (One)”.