What is the scientific definition of "brain training"? A lot of companies are selling products under the heading of "brain training", and "brain exercises", "brain games" such as Nintendo DS Brain Training, Lumosity, and a host of other companies.

I'm highly sceptical that such exercises have any generalised effect beyond improving the task that is being practiced and possibly some very limited form of generalisation. Nonetheless, I was curious about what makes an exercise or a program of activities "brain training". 


What is the scientific definition of "brain training"?
Alternatively, what definitions have been proposed by researchers or very popular providers of brain training?


Such a definition would presumably make clear how daily living is different to brain training. It would also hopefully clarify whether daily cognitive activity like doing a crossword, answering a question on stack exchange, making a shopping list, or playing a computer game is brain training or not.
I think in most scientific contexts, "brain training" refers to working memory training. Scott Barry Kaufman wrote a nice article entitled In Defense of Working Memory Training. Hulme and Melby-Lervag wrote a meta-analysis about working memory training. They note that these exercises would not generalize to other mental skills such as reading comprehension, word decoding, and arithmetic. However, they did find that these exercises did improve short term verbal and visuospatial spatial working memory skills. 

Kaufman argues that one should not dismiss working memory training as not improving intelligence. It may improve one aspect of intelligence (i.e. working memory). Note that according to may researchers, intelligence is comprised of many facts. Also, the age of people participating in brain training should be taken account. For example, very young people and very old people may benefit more than "normal age" people. Likewise, personality traits such as conscientiousness and neuroticism should be taken into account.

Kaufman also notes that mindfulness meditation and other "whole body" exercises would be more beneficial than specific working memory training exercises. 

Sources

Is Working Memory Training Effective? A Meta-Analytic Review by Monica Melby-Lervåg
and Charles Hulme. Developmental Psychology © 2012 American Psychological Association
2013, Vol. 49, No. 2, 270–291.

A Dynamical Model of General Intelligence: The Positive Manifold of
Intelligence by Mutualism by Han L. J. van der Maas et al. Psychological Review 
2006, Vol. 113, No. 4, 842–861.