Xi is one of the least visible letters of the Greek alphabet in everyday modern use, appearing mainly in specialised mathematical notation, where it commonly denotes a random variable in probability theory or an unspecified point in certain calculus theorems, alongside its use for a Bayer-designated star sharing the same ranked naming scheme as its neighbours. Its most widely reported recent appearance came in 2021, when the World Health Organization, working through its Greek-letter naming scheme for coronavirus variants, announced that it would skip xi, along with the preceding letter nu, when assigning the next variant name; WHO officials stated that xi was skipped specifically to avoid inadvertently naming a serious disease variant after a common surname, a concern that had nothing to do with the letter's ancient linguistic history, covered elsewhere in this atlas, or with its Semitic root samekh, meaning a prop or support.
The decision drew considerable public commentary at the time given its evident political sensitivity, making that single naming decision, rather than any classical or scientific use, xi's best-known moment in recent public discussion. In the Milesian numeral system, discussed elsewhere in this atlas, xi carries the value sixty, a use so specialised that most readers encounter the letter only through mathematics or through the 2021 naming controversy rather than through any everyday context.