Is Wikipedia's page on the sigmoid function incorrect? Is Wikipedia's page on the sigmoid function incorrect?
It states that:

A common example of a sigmoid function is the logistic function

From my knowledge of machine learning, I thought that "the sigmoid function" is defined as the logistic function, $$\sigma(z) = \frac {1} {\left(1 + e^{-z}\right)}\text{.}$$ I have never seen or heard the phrasing that the logistic function is a type of sigmoid function.
Furthermore, that Wikipedia page says that other examples of a sigmoid function are the tanh and arctan functions. Again, I've never seen tanh nor arctan described as a type of sigmoid function.
These functions are considered to be peers, usually in a context like:
We can use various non-linear functions in this neural network, such as the sigmoid, tanh, and ReLU activation functions.
What am I missing here? Is the Wikipedia article correct or incorrect? I find that Wikipedia is usually accurate for math terms.
 A: I believe one more answer, specifically addressing your points as they currently stand (Revision 11) and comments is warranted.

Is Wikipedia's page on the sigmoid function incorrect?

No. In some communities, specifically Machine Learning, some (maybe even most?) people use the term "sigmoid function" in a different, more limited sense, as a synonym for the logistic function. But, not the whole community does so, Machine Learning is not the only community using the term, and Wikipedia is not an encyclopaedia of Machine Learning. It addresses a broader audience, which uses a different terminology and has been using it probably before Machine Learning has been invented.

I have never seen or heard the phrasing that the logistic function is a type of sigmoid function.

Wikipedia also doesn't use this exact wording, so you seem to be misquoting it. But, semantically, considering the logistic function just a member of the sigmoid family is not at all uncommon, not even in the ML community. See for example:

A Sigmoid function is a mathematical function which has a characteristic S-shaped curve. There are a number of common sigmoid functions, such as the logistic function, the hyperbolic tangent, and the arctangent.

Examples of such usage in other communities have been given in other answers and comments.

These functions are considered to be peers, usually in a context like:


We can use various non-linear functions in this neural network, such as the sigmoid, tanh, and ReLU activation functions.

Again, this is just ML-specific lingo and even there the situation seems not to be so clear-cut. For example, in Python's Scikit-learn (an ML library!), the neurons in a multi-layer perceptron can have identity, logistic, tanh, or relu activation functions, but not "sigmoid".
From the comments:

I work in applied ML, and I would probably be knocked down by my peers if I said that I used a sigmoid function in my neural network instead of being more specific and saying that I really used tanh.

When in Rome, do as the Romans do. But, that goes in both directions. Machine Learners, when addressing other audiences, should be specific and use "logistic function" instead of "sigmoid".

I posted this question on a ML site, purposefully to limit the scope of the audience to people in my field.

Cross Validated's scope is broader than just Machine Learning:

Cross Validated is a question and answer site for people interested in statistics, machine learning, data analysis, data mining, and data visualization.

A: The unsatisfying answer is "It depends who you ask." "Sigmoid", if you break it into parts, just means "S-shaped".
The logistic sigmoid function is so prevalent that people tend to gloss over the word "logistic". For machine learning folks, it's become the exemplar of the class, and most call it the sigmoid function. (Is it myopia to call it the sigmoid function?) Still, there are other communities that use S-shaped functions.
A: As Arya said, it depends who you ask, but this is not specific to Machine Learning, and even in Machine Learning the situation is not consistent (or not consistently bad). Bishop, for example, uses the term "logistic sigmoid function" and Jordan used "logistic function" already in 1995. In Statistical Mechanics, on the other hand, people are likely to call it the "Fermi-Dirac distribution/function". In some fields of biochemistry, including toxicology, you'll meet the same thing under the name "Hill equation". Etc.
It is IMHO important to remember that these are only names (words) used for describing a mathematical concept. Words is what people use to communicate, for example ideas and methods. As long as all participants of the communication understand what concept they are talking about, it doesn't really matter what words they use for it. Communities develop to a large part independently from each other (otherwise they would form a single community) and develop field-specific "dialects".
As a related example, the words "weight" and "bias", in the context of neural networks (and, through historical development, support vector machines) have completely different meanings from those used in statistics, but there is historical/field specific justification for using them.
Update: Actually, neural network pioneers commonly use "logistic function" or "logistic neuron": Hinton, Rumelhart and McClelland (also here), Sejnowski etc.
Update 2: Also, one might as well ask: "Is RBF just the Gaussian function?". For some reason, equating the two on CV doesn't seem to cause nearly as much commotion as your question.
A: It should be clear that the mentioned Wikipedia page has some terminology issues.
Wikipedia's statement

A common example of a sigmoid function is the logistic function

and assertions that these functions are examples of sigmoid functions

are confusing at best.
The logistic function is not a type of sigmoid function. The sigmoid function is the logistic function. Likewise, the tanh function is not a type of sigmoid function.
Stanford's Andrew Ng states the terminology concisely in this video on neural network activation functions. This is the correct terminology to use if you are working in this field.
https://www.youtube.com/watch?v=P7_jFxTtJEo
