What is the meaning of 'Marginal mean'? My problem with understanding this expression might come from the fact that English is not my first language, but I don't understand why it's used in this way.
The marginal mean is typically the mean of a group or subgroup's measures of a variable in an experiment, but why not just use the word mean? What's the marginal here for?
See the definition of marginal from wiktionary. 
 A: Perhaps, the term originates from how the data is represented in a contingency table. See this example from the wiki.
In the above example, we would speak of marginal totals for gender and handedness when referring to the last column and the bottom row respectively. If you see the wiktionary the first definition of marginal is:

  
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*of, relating to, or located at a margin or an edge
  

Since the totals (and means if means are reported) are at the edge of the table they are referred to as marginal totals (and marginal means if the edges have means).
A: I'd assume it means the sample analogue of the marginal expectation $\operatorname{E}(X)$, as opposed to the sample analogue of a conditional expectation $\operatorname{E}(X \mid Y)$, where $Y$ could be anything.
A: Can't add it as a comment, so here it comes as an answer: 
As user28 already said, the marginal mean refers to the mean of a factor level, which - in a cross-table - is at the table's margins, hence the name marginal mean. 
Why this term is not entirely redundant? "Mean" could mean just any mean, e.g. the mean of all right handed men in the example of user28. By saying "mean of factor A" you should mean the mean of all levels of factor A, but you could mean (or be misunderstood as meaning) the mean of one level of factor A. Using the term "marginal mean of factor A" makes it unambiguously clear what you mean.
