A correspondent writes from Brazil to say that it is his impression that the usage label Colloquial has virtually disappeared from English language dictionaries, being replaced by Informal. Is this true, and, if so, why? He notes that colloquial is still very much alive in Brazilian lexicography.
It does seem to be true. I can't remember the last time I saw colloq in a dictionary. I think the reason is that many of the early terms changed as stylistic labelling in dictionaries became more sophisticated - a consequence of the more linguistically informed lexicography of the past few decades. In the old days, it was assumed that the only words worth including in a dictionary were words from written English, assumed to be formal and standard. Anything from speech was then conveniently dubbed ‘colloquial’.
But once it was realized that both speech and writing contained both formal and informal varieties, the term colloquial was not so effective, because it lacks an antonym (apart from non-colloquial, and the like). Formal and informal, however, made a neat terminological contrast, and allowed in the notion of a scale of formality. I remember opting for this change myself, in some early stylistic work in the 1970s. In Investigating English Style (1969) Derek Davy and I used (in)formal mainly, with some reference to colloquial, but by the time we wrote Advanced Conversational English (1976), (in)formality was our norm.
Informal is also wider in its application, being equally at ease in relation to both speech and writing. Colloquial is very much bound up with speech: could one talk of 'colloquial writing'? And even in relation to speech, informal is more useful, as it enters more readily into collocations - one can easily say informal conversation, but colloquial conversation is a bit odd. So I'm not surprised really to see the terminological shift in dictionaries - at least, in monolingual English dictionaries. I've just looked at some bilingual dictionaries, and I see colloquial and formal used in a few cases, perhaps because the choice of terms has been influenced by a different terminological tradition in the other languages.