alphabet; ABCs
Refers to the set of letters from A to Z, or the basics of reading and writing.
I teach children the ABCs.
basics; fundamentals; rudiments
Figurative use: the elementary principles or first steps of a subject. Often used in the pattern 〜のＡＢＣ.
I learn cooking from the basics.
This book explains the basics of management in an easy-to-understand way.
ABC is a costing method that allocates overhead costs by activity.
The ABC is considered one of the first electronic computers.
I learned the alphabet when I was a child.
ＡＢＣ implies the absolute starting point one must learn first, whereas 基本 refers more generally to the core principles that remain important throughout study.
ＡＢＣ and 初歩 both mean the rudiments of a subject, but 初歩 is a plain Japanese word while ＡＢＣ carries a slightly more colloquial, borrowed feel.
From English 'ABC', borrowed as a loanword. The katakana reading エービーシー reflects the English pronunciation.