A word in French or Spanish (in Spanish, it's tú) meaning, literally, "thou." It's the singular familiar second person pronoun in the nominative case. Many languages use plural, third person, or other forms for formal you.

This kind of thing is virtually universal in Europe, even in the unrelated languages Finnish, which uses second-person plural sinä, and Hungarian which uses maga with third person, and Basque, which uses plural zu as formal singular and has created a doubly-plural from zuek for the plural. I don't know when it began, though I am led to understand the first known occurrence of plural for singular is in Latin in the poetry of Catullus. English has actually lost the formal/familiar distinction by virtue of losing the old singular thou entirely.

The distinction is commonly called T/V by linguists (yes, that's what we have fun doing, we talk about T/V and if we're Latin-lovers we might even try it out) because of pairs like French tu ~ vous, and Russian ty ~ vy. However, note that Spanish plural vosotros, -as is the T (familiar) form, since the V (formal) is usted(es).

As well as the shift from singular to plural (and no I don't know why that should be honorific or polite), many European languages use roundabout third-person constructions, like the English "Does Modom wish to pay for Modom's hat now?" and "Your Majesty is like a stream of bat's piss". In Portuguese they say o senhor 'the gentleman', a senhora, and so on; in Romanian it's domneavoastra 'your lordship'; Spanish usted comes either from Arabic ustad 'master' or from vuestra merced 'your honour(?...roughly).

In Italian the noun for 'excellency' being feminine, the pronoun lei 'she' is also used for 'you' (singular).

In German they use Sie 'they', but I don't know why.

Elsewhere in the world you get somewhat similar-looking systems, but actually more complex. Both Japanese and Indonesian have several actual pronouns, or you can use titles such as sensei or Indonesian Bu 'Mother' (to an older woman), Tuan 'Lord', or you can use the names, as Tanaka-san or Abdul, with significant etiquette behind the choice.

Although linguisticians love sticking these phenomena in handy categories, for practical purposes it should be noted that the degrees of familiarity at which one switches from the higher to the lower register form vary quite a lot between different languages and dialects. Thus in standard French the vous form is ubiquitous between adults even in quite close relationships, while Italian lei and Dutch U give way to tu and jij at a fairly early stage of acquaintance.

Furthermore some languages have more than just the two levels: European Portuguese probably scores highest with a scale that runs from:

vossa excelencia
o doutor/a ingenhera
o senhor/a senhora
você
tu

There have been attempts to stamp out the distinction: the Italian fascists brought in the use of the unmarked voi in an attempt to recreate the Latin tu (which was just a singular unmarked form); like most government imposed linguistic reforms, it didn't work (exceptions are the terminology of Italian football and UK decimal currency), athough singular voi is still used in some dialects as an intermediate between tu and lei or as the polite form..