  Altermagnetism, recently spotlighted in condensed matter physics, presents
captivating physical properties and holds promise for spintronics applications.
This study delves into the theoretical description and categorization of
two-dimensional altermagnetism using spin group theory. Employing spin-group
formalism, we establish seven distinct spin layer groups, extending beyond the
conventional five spin Laue groups, to describe two-dimensional altermagnetism.
Utilizing these findings, we classify previously reported two-dimensional
altermagnets and identify novel materials exhibiting altermagnetism.
Specifically, monolayer MnTeMoO$_6$ and VP$_2$H$_8$(NO$_4$)$_2$ are predicted
to be two-dimensional altermagnets. Furthermore, we scrutinize their
spin-momentum locking characteristics through symmetry analysis and density
functional theory calculations, substantiating their altermagnetic properties.
