Readability is very important to educators, and particularly to children’s writers. We try to use vocabulary that students won't get stuck on, preventing them from concentrating on the concepts that are being explained. For this reason, we often use categories of living and non-living when describing the biotic/abiotic dichotomy in science. Biotic means living, and abiotic means non-living.
However, this language sometimes leads into a misconception of associating living with the state of being alive. Dead stuff is still biotic, or living if we’re using simplified language. Lacking signs of life does not make something abiotic. (Texting teenagers, for example, may not appear alive but still remain classified as biotic.)
This becomes less problematic if we apply the categories to groups, rather than to individuals. Lumber doesn’t resemble a tree, but it is still biotic. Trees are biotic components of the world. Similarly, an infertile individual does not get classified as non-living (abiotic). They are classified along with their whole species, as biotic.
Neither is there is a third, “once-living” category—no “once-biotic.” Sedimentary rocks were formed from the remains of living things millions of years ago. In that sense, the components of rocks were once living, but rocks are classified as non-living.
Biotic: things that reproduce, grow, and die, and the waste from these things
Abiotic: not derived from biotic things
Photo via US Dept Agriculture, used under CC BY-2.0 license.