In Foldit, the terms "residue" and "segment" are used more or less interchangeably. Both normally refer to the amino acids that make up a protein. The scripting functions in the Foldit Lua interface typically refer to segments. Puzzle titles often refer to residues, as in "85 Residue Monomer Design".
In some puzzles, one or more segments represent "ligands", which are atoms or small molecules that interact with the protein in some way. This is an example of a Foldit segment which is not a residue.
"Residue" in this sense technically means "amino acid residue". See the IUPAC definition for "amino acid residue" for details.
The sidechain of a residue may still be acid, however.
(A previous version of this article incorrectly defined "residue" as usually referring "exclusively to an amino acid's sidechain". Chemical diagrams often use an "R" for the sidechain, but "R" does not mean "residue" in the Foldit sense. See this Wikipedia explanation of the "R" notation.)