here is the definition from DIAC:
<b>Other Dependent Relatives</b>
Other relatives of you or your partner may be considered in the application if they meet all of the following:
they have no other relative able to care for them in their own country and they either:
are not currently married, engaged or in a de facto relationship
are usually resident in your household
rely on you for financial support for their basic needs and you have supported them for a substantial period
rely on you more than any other person or source.
Example: An aged, unmarried relative.