Vitalik Buterin, one of Ethereum’s co-founders, is credited with this trilemma, and this (AFAICT) is where he first posted about it:
The trilemma claims that blockchain systems can only at most have
two of the following three properties:
* Decentralization (defined as the system being able to run in a
scenario where each participant only has access to O(c) resources,
i.e. a regular laptop or small VPS)
* Scalability (defined as being able to process O(n) > O(c) transactions)
* Security (defined as being secure against attackers with up to O(n) resources)
In the rest of this document, we'll continue using c to refer to the size
of computational resources (including computation, bandwidth and storage)
available to each node, and n to refer to the size of the ecosystem in some
abstract sense; we assume that transaction load, state size, and the market cap
of a cryptocurrency are all proportional to n. The key challenge of scalability
is finding a way to achieve all three at the base layer.