There is not a strict division of register into two classes, 'formal' and 'informal'.
The informal language continuum describes how language use varies
from very casual, personal communication to more formal, impersonal
communication. Instead of a strict rule of "right" or "wrong," it
shows a spectrum of language options used in different contexts, such
as from a text message to a published book. Examples of features that
change along this continuum include the use of contractions, slang,
and first-person pronouns, all of which shift based on the level of
formality required. [Google]
An informal language continuum is a spectrum of linguistic styles
that ranges from very casual and intimate to moderately formal,
without a clear dividing line between them. Unlike the strict rules of
formal language, the "rules" of informal language are determined by
the context, relationship between speakers, and other social factors.
A speaker's position on the continuum can shift instantly depending on
the situation. The concept is also called the "five levels of
formality" or "registers" by linguists such as Martin Joos. These
levels demonstrate how different situations require different
linguistic choices, moving from highly personal and relaxed to more
detached and professional. [Google]
One approach is to divide registers into a five-level formality model:
The five levels of the continuum
1. Intimate
This is the most casual and private register, used among close family
members, partners, and friends. Characteristics: Simple, often
incomplete sentences. Uses pet names, abbreviations, and inside jokes.
High use of slang and colloquialisms.
Example: "Hey, hon. Need anything from the store?"
2. Casual
This register is used among friends, acquaintances, and coworkers.
It's conversational and relaxed, but less personal than the intimate
level. Characteristics: Uses contractions (e.g., "can't," "I'm"). May
use slang but less exclusive inside jokes. Loose sentence structure.
Example: "Hey, what's up? Wanna grab a coffee after this?"
3. Consultative
This is the standard, neutral level of formality used for most
professional and educational settings. Characteristics: Interaction
is professional but not stiff. Often involves conversations with a
teacher, manager, or client. May use specific jargon related to the
professional context.
Example: "Good morning. I was hoping to ask you a few questions about
this assignment."
4. Formal
This register is impersonal and highly structured, with little to no
interaction between speaker and audience. Characteristics: Complex
grammar and vocabulary. Avoids contractions, slang, and first-person
pronouns. Used in lectures, official reports, and ceremonies.
Example: "It is my great honor to address this distinguished student
body tonight."
5. Frozen
This is the most formal and fixed style of language, reserved for
texts that do not change. Characteristics: Fixed, static language.
Uses archaic vocabulary in some cases. Found in legal documents,
religious texts, and wedding vows.
Example: "Dearly beloved, we are gathered here today..."
Factors that influence position[ing] on the continuum
Several factors can cause a speaker or writer to shift their language
along this continuum:
- Audience: The speaker's relationship with the listener(s) is the most significant factor. You would talk differently to your partner, a
friend, a coworker, or a judge.
- Purpose: The goal of the communication dictates the register. A job interview requires a more formal style than a social chat. Setting:
The physical environment also plays a role. You would speak
differently at a sporting event versus a lecture hall.
- Medium: Texting and social media tend toward the casual end, while academic papers and official emails are more formal.
- Social context: Social dynamics, power structures, and cultural norms all influence what level of formality is considered appropriate. [Google]
So, using this model, colloquial usage overlaps largely with the 'intimate' and to a lesser degree with the 'casual' registers. It is not really appropriate in settings requiring 'consultative' or higher levels of formality.
Applying the tests for choosing optimal register, I'd say 'a low-hanging fruit' is acceptable in all but the most formal of documents, 'circle back' likewise, while 'ain't it' and 'kinda' would be in the least formal ('intimate') class (while not necessarily referring to conversations one would normally describe as being 'intimate'). 'Spill the beans' and 'get the ball rolling' are in the 'casual' class, while 'strong coffee' is unmarked, being equally acceptable across all levels of formality.
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As jsw29 sagely points out, while 'colloquial' is an imprecise term overlapping the intervals of formality outlined in the model above, 'jargon' seems orthogonal or nearly so. It does describe a register (or rather registers), but these relate to domains within which various subject-specific usages are typically found. The examples 'a low-hanging fruit' and 'circle back' are hardly confined to the 'corporate domain' (though the former is quite probably most commonly used in business English). Perhaps 'punt' (postpone) is more domain-specific.
As far as the acceptability of jargons is concerned, they are usually fine– advantageous or even needful, even – within a community well versed in their use ... but to be avoided when attempting to communicate with outsiders. There is not the socio-economic tie-in with users that registers of different degrees of formality have, each domain forming its own sphere of usage. Because a jargon has a distinct vocabulary, it is unlikely that more than one way of expressing most things needing these additional terms will exist, thus making formality levels correspondingly unlikely. But as jargons borrow from the standard language, some terms may well seem more informal than others.
