According to Wikipedia reflexivity is defined as follows:
Reflexivity refers to circular relationships between cause and effect. A reflexive relationship is bidirectional with both the cause and the effect affecting one another in a situation that does not render both functions causes and effects. In sociology, reflexivity therefore comes to mean an act of self-reference where examination or action "bends back on", refers to, and affects the entity instigating the action or examination.
To this extent it commonly refers to the capacity of an agent to recognize forces of socialization and alter their place in the social structure. A low level of reflexivity would result in an individual shaped largely by their environment (or 'society'). A high level of social reflexivity would be defined by an individual shaping their own norms, tastes, politics, desires, and so on. This is similar to the notion of autonomy. (See also: structure and agency, social mobility)
In Economics reflexivity refers to the self-reinforcing effect of market sentiment, whereby rising prices attract buyers whose actions drive prices higher still until the process becomes unsustainable and the same process operates in reverse leading to a catastrophic collapse in prices.
Soros has been a big proponent of reflexivity, and here is a lecture from him with Q&A discussing the subject and how it impacts investing: