constituent (n.)

1620s, "one who appoints or elects a representative," from Latin constituentem (nominative constituens), present participle of constituere "to cause to stand, set up, fix, place, establish, set in order; form something new; resolve," of persons, "to appoint to an office," from assimilated form of com-, here probably an intensive prefix (see com-), + statuere "to set," from PIE root *sta- "to stand, make or be firm."

The notion is "to make up or compose" a body by appointing or electing a representative. Meaning "voter in an election to a public office" is from 1714. Meaning "that which constitutes as a necessary part, a formative element" is from 1756.

"essential, characteristic, existing as a necessary component," 1660s, from Latin constituentem (nominative constituens), present participle of constituere "to cause to stand, set up, fix, place, establish, set in order," from assimilated form of com-, here probably an intensive prefix (see com-), + statuere "to set," from PIE root *sta- "to stand, make or be firm."

Meaning "that appoints or elects a representative to a public office" is from 1769, from the political sense of the noun.