Shareware is software that is freely shared on a “try before you buy” basis.
Often it is shared as a trial version that,  when paid for, can be unlocked with a license key to access all the features of the program.

The word “shareware” became popular in 1983 with the word processor “PC-Write”, created by Bob Wallace.
Bob Wallace said that he got the idea for shareware “to some extent as a result of my psychedelic experience” and that he first saw the word in a computer magazine somewhere in the 70ies.
The word shareware was a reaction to “freeware”, a term that was coined a year earlier by Andrew Fluegelman, creator of the telecommunications program “PC-Talk”.
Freeware is software that is freely distributed without any pay asked for it, ever.
Or as Fluegelman phrased it: “as an experiment in economics more than altruism”.

Shareware became very popular in the 1980ies and the 1990ies as the software was distributed by floppies and BBS networks, which both are a lot more limited than the internet we know today.
Today the exchange of software is much easier and faster.
So it is easier to distribute 2 versions of a software in the form of the free version and the paid version. Which leaves the software less vulnerable for people who cracked keys on shareware and thus depriving the programmer of their reward for all their hard work.

• https://sciencelens.co.nz/2012/12/08/international-shareware-day/
• https://www.cision.com/2010/11/international-shareware-day/