Singular to the polymer
's plural, the monomer is the unit, the fragment. A seed is a monomer in a seed pod
and ceiling tile (look up!) is a monomer in the lattice
Although polymer's monomers are connected by strong atomic bonds, protein monomers aggregate in a much less permanent way. Like cheap self-assembly furniture, a multisubunit protein (a monomer is also a sub-unit) can fall apart. Or change shape, or add more monomers, or fluctuate between states.
Your SO is a monomer in the dimer formed by your relationship, and might break away at any moment. Hug!