Well I worried that this would be too simple a question, but looks like it’s not that easy.
How does &c replace etc? I read it as “and c” and spell out etc.
Etc is an abbreviation for et cetera. The et is Latin for ‘and’. And the & symbol looks rather like the characters E t c smooshed together.
Originally it was the e-t ligature in lead type – a single piece of type containing 2 characters that are commonly used together, so the type designer has created a joint character for them in his typefont. Now it’s most commonly seen in greek/latin words, like the a-e as æ in æsop.
Numeral is a word class. At least that’s what I was taught in my linguistics classes! 
Except a ‘5’ looks like an ‘S’. Whereas the ‘7’ in ‘Se7en’ at least looks like a ‘V’ that’s tipped over.
‘Thr33’ would work.
As would ‘Ei8ht.’
And ‘Zer0’ I suppose.
The rest, like ‘5ive’ and ‘12elve’ are pushing it. They can get away with it by forcing it, but it’s not elegant or logical.
Interestingly, telegraph offices seem to have considered numerals as full words, so 42 is 2 words, just like Forty Two. So, despite all of the wacky ciphers and message shorteners that they used back in the day, replacing words with numbers wasn’t one of them.
Princely!
(Purple?)
Then they taught you wrong.
2 and 4 represent the words “two” and “four” in English.
In Spanish they represent “dos” and “cuatro” respectively; in Russian “два” and “четверка”; in Magyar “kettő” and “négy”.
I think jayjay’s answer that they are a subset of rebus is correct – in a rebus an iconic image stands for a homonymous word (or occasionally a letter), e.g., a picture of a honeybee for “be” or of a lynx for “links”. “To”, “too”, “for”, “ate”, etc. are represented by the numerals representing their homonyms. (I recall someone trying [unsuccessfully] to get the license plate/tag number 6UL*DV8. :))
I’m not saying you’re wrong, but I’d like a citation from a reputable journal if that’s alright. I probably have access via my university if it’s not open to the public.
My problem here your definition of a word. Words exist independently of alphabets. “2” and “4” are signifiers of the concept I have in my head of “2” and “4”. I don’t need to know how to spell them (or even know they can be written out using an alphabet) to think them or say them. “2” and “two” are 2 different representations of the same thing. My basic point being that ‘2’ doesn’t necessarily represent ‘two’, there is no need for a middle man.
Whenever we’ve used grammar models to describe sentences like “I have 2 goldfish” we’ve described ‘2’ as a determiner, quantifier and a numeral.
Ok, how about using letters to replace words? Does that have a name? Like Toys 'R Us. I think using numbers can be counted as part of that.
If it’s used to shorten, it’s all abbreviation. Contraction is just a specific form of abbreviation. Abbreviate merely means “to make brief.”
Prince-ification?
Toys “Я” us to be precise. 
Toys Ya Us. Darn Russian sleeper agents…
Wrong kind of toys, I should think.
It looks even more like Et smooshed together, which is why it still has a c after it.
Obligatory link to Victor Borge’s Inflationary Language.
I am with calling it a for of rebus.
I agree. It’s a form of rebus: using symbols or pictures to represent words (or parts of words).
Note that a rebus crossword puzzle is one that uses the gimmick of having symbols (including numerals) represent parts of words in the grid—though in that case they may stand in for either the sound or the spelling.
I like that word.
I disagree. The “7” is effective–and novel/arresting–because it is neither iconically related to a tipped over “V” (although it is unfair to argue with whatever you see in a grapheme), nor that its initial sound fits. Eg, for the latter, “The Jackson 5ive” does, with the additional fillip of their being five members of the group. Just lucky/nice that it works in English–as needs must any of the sonic plays in any language. Perhaps someone could post some from other language. [Wow, "fillip and “needs must” in one sentence! Weird sentence construction is worth it just for that!]
This is implicit in Polycarp:
However, I must take issue with the rest of his/her reply, along with the posters who concurred with him
The semiotic status of a grapheme in a literate community is iconic (pictorial, recognized as such), symbolic (purely arbitrary), or any combination of the two. (The original presentation of this idea had the two terms on an x-y plot.)
In a rebus, the iconic is usually the point. In any number of alphabetic letters, the soundsense (to use a Joyce word) is symbolic. “R” sounds like <make “R” sound> why…cuz. Symbolic. Frenchmen this way, Anglos that, Japanese the other way.
Now, to iconic. First, numerals. I am not a specialist in this, and cases have been made one way or the other, but the iconic nature of “1” is clear; “7” is not.
So, iconic, symbolic, and “concept” (what the linguistics guys upthread were discussing). Consider the letter “bet” in Hebrew, famously an icon of a house–a word, or concept.
“Bayit,” the Hebrew word for the letter with different vowels, means “house” in Hebrew. Is it a “Jackson 5ive” word, or even a simple redundancy?
I’d like to think of others in Hebrew or similar ones in other scripts, but my brain is too tired to create some.