An activation function is a function used by an artificial neuron (represented as a node in a graph) of an ANN (i.e. the weighted directed graph) which produces some output given one or more inputs to the neuron. There are many different activation functions.
In this answer, an activation of a neuron is defined as a function of its input: specifically, it's a transformation of this input, which is performed via an "activation function".
Finally, the author of the same answer also says that the activation of a neuron can be interpreted as an internal representation of the input.
Is there any difference between an activation and an action function of an artificial neuron in an ANN? If yes, which one(s)?
I suppose these are just two terms that, in practice, refer to the same thing, i.e. a mathematical function, but, for some reason, in theory, some people use these two terms. If I am correct, what's this reason?