Hello Tom
There are various terms used to refer to similar technologies from different vendors. Chassis Aggregation is a term used by Cisco to refer to the architecture of using two chassis switches to act as a single aggregated switch. The actual technology that is used to achieve this architecture is Virtual Switching System or VSS. Chassis Aggregation architecture can only be achieved between chassis-type switches.
There are other aggregation technologies for both chassis and non-chassis switches. You can find out more about these at this NetworkLessons note.
As for [MLAG](Take a look at this note for more info as well.), from the point of view of Cisco, this is simply another term used to refer to the application of LAG across switches. The specific technologies that achieve this are those mentioned in the NetworkLessons note above. The terminology that each vendor uses should be understood within the context of their documentation.
I hope this has been helpful!
Laz