This is the 4605th most frequent Arabic word.
إحداهما
"One of them."
Refers to one item out of two specified in the previous context, here the two books.
اشتريت كتابين، إحداهما عن التاريخ والأخرى عن العلم.
I bought two books, one of them about history and the other about science.
Indicates a selection of one specific apartment from two choices.
لدينا شقتان، إحداهما تطل على البحر.
We have two apartments, one of them overlooks the sea.
Highlights one out of two relatable aspects or items, which adds specificity to the description.