A blend word is a word made up of parts of two (or more) other words, and whose meaning combines the meanings of the two other words.
e.g. Breakfast + Lunch → Brunch
How and why do we make them?
Blend words are usually made up of the beginning of the first word and the end of the second word (like brunch). We can also use the beginnings of two words, e.g. email from “electronic + mail”.
Blend words are popular partly because they allow us to condense language, e.g. by merging two syllables which sound the same. This is called haplology and is even present in the word England itself, which used to be Engla Land, so originally England was a blend word! Sometimes the blend words form a family based on a common component, e.g. the -exit family: Brexit ← Britain + exit; Grexit ← Greece + exit; and many other possibilities, even Texit ← Texas + exit.
So one blend word might inspire other similar ones, e.g. alcoholic inspired workaholic and shopaholic. Other families include -ware (hardware, software, freeware, etc) and -nomics (economics, Freakonomics, Raeganomics, Trumponomics, etc).
Some words may not form a simple blend. When combining man + explaining → mansplaining (a man patronising a woman), the -x- has changed to an -s- to avoid a “non-English” sound sequence of n + ex + s + p + l, which would be quite a mouthful.
Blend words clearly can increase efficiency, but acronyms can be even more efficient, e.g. YOLO (=You Only Live Once) or LOL (laugh out loud) saves time to say when spoken as an acronym, which is increasingly common.
Evidence of blend words being used in English
The popularity of blend words is partly due to the fact they are easy to make up, and so a lot of them are introduced in the media and online, e.g. about celebrity couples (e.g. Brad [Pitt]+ Angelina [Jolie]→ Brangelina), or the cronut, part croissant part doughnut. For this reason it is hard to know how many there are in English, as new ones could be coined as you are reading this, but a look at the Wikipedia list contains about 800 blend words, some more famous than others, and indeed the word Wikipedia itself.
source: Cambridge English
