Kin selection extends the obvious logic of parental sacrifice to relatives. [definitional]
The idea of 'kin selection' is an extension of the intuitively obvious point that animals will often put themselves at risk and do things that are bad for their health in the production and care of offspring.
DEFINE: Clarifies the concept of kin selection by grounding it in the intuitive behavior of parental care.
Diego Gambetta, Trust - Making and Breaking…, loc. 311