Graphical depiction of the ways an entity can exist and the transitions between them.
State Diagrams give an abstract depiction of the different modes in which an entity or object can exist, and what the allowed transitions are (optionally annotated with mechanisms and/or names for those transitions). There are State Diagrams in UML, SysML, and the sole diagram type in OPM is arguably best described as a hybrid State Diagram/Block Definition Diagram.
Intensive dives into state diagrams gets you into the realm of graph theory in mathematics, like Petri Nets.
Example
Using Mermaid to depict Ice Cream.
stateDiagram-v2
s1: In carton
s4: On_Floor
s5: Consumed
s1 --> s4 : server dropping
s1 --> In_Cone : serving
In_Cone --> s4 : customer dropping
In_Cone --> s5 : customer eating
state In_Cone {
s2: Firm
s3: Messy
s2 --> s3 : melting
}