A sysregressor is a headmate who fulfills a similar role as a syskid but is an age regressor instead of a child.
This is not the same thing as as an age slider, because an age slider's actual in-space age changes, whereas an age regressor adopts the mindset of a child without necessarily becoming the age of one in-space. Additionally, some age sliders may not see themselves as syskids or having the role of one.
This is also not the same thing as being an age regressor who is in a system. They have to feel that their age regression is similar or comparable to being a syskid/little/child headmate/etc.
What makes it similar to being a syskid is up for interpretation and ideas about that can vary between different systems or different headmates in the same system. Some systems could simply see that as age regression as a system role, or some systems may have more specific criteria.
While this definition has been written with age regressors and syskids in mind, this can also apply to pet regressors and syspets while using the same term.