A definition that I like to use is the one that O. Brown and P. Eremenko [Ref. 1] provide in his work “Fractionated Space Architectures: A Vision for Responsive Space”:
A responsive space is the capability of space systems to respond rapidly to uncertainty.
Apart from responsiveness needs, complexity in missions is increasing: high data volume demand, better resolution, near-real time data availability between others … thus the large and monolithic spacecraft could not cover all these requirements using the classical paradigm. In order to solve these needs,
A shift is taking place from large and costly spacecraft carrying multiple payload capabilities to distributed low-cost small satellite systems working together to achieve a service or a need and generating multi-mission opportunities.
Some common architectures are constellations, formations, clusters and trains; but satellite mission concepts are becoming convoluted and require outstanding capabilities such responsiveness as commented above, flexibility, scalability, real time fault-tolerant resource sharing and maintainability to name a few.
Read more at PULSE: https://www.linkedin.com/pulse/thinking-small-powerful-distributed-satellite-systems-samsó-msc?trk=mp-reader-card