An Alphabetacy is a style of Self-Imposed Challenge (with a corresponding genre of Fanfic) specific to the Sims player community: In a variety of what is called the "Legacy Challenge", the goal is to create a multigenerational family of sims in which each generation is given proper names starting with letters of the alphabet in order -- for instance the first generation all have names starting with "A", the second have names starting with "B", and so on.
In keeping with the usual Legacy Challenge rules, the family is started on a large lot with a single Sim in abject poverty, who must find a mate, grow a fortune, build a house, and start a new generation, who must then maintain the family home and produce the next generation of heirs. Points are awarded (or subtracted) based on numerous factors, including number of children in each generation, sets of twins, intervention by Social Services and so on. The true nature of the challenge comes from the difficulty in maintaining a family line over so many generations -- the basic Legacy Challenge settles for a mere 10 generations as a goal, but an Alphabetacy dares the player to go all the way to the 26th generation.
The Alphabetacy challenge has been defined for Sims players since at least 2007 and probably earlier.