What is dyscalculia?
Dyscalculia is a specific learning disability (or difficulty) in mathematics. It was originally defined by the Czechoslovakia researcher Kosc , as a difficulty in mathematics as a result of impairment to particular parts of the brain involved in mathematical cognition, but without a general difficulty in cognitive function. This is the same definition that researchers in cognitive neuroscience use today.
Two other common terms come from different backgrounds but essentially mean the same thing; these are "Mathematics Disorder" (a diagnosis in the DSM-IV, a manual used by psychiatrists), and "Mathematical Disabilities", an educational term primary used in North America.
What all definitions have in common is 1) a presence of difficulties in mathematics, 2) some degree of specificity (ie. lack of across the board academic difficulties) and 3) the assumption that these are caused in some way by brain dysfunction.
Note that here dyscalculia refers to what is strictly called "developmental dyscalculia". There is another type of dyscalculia called "acquired dyscalculia" which is acquired (usually in adults) as a result of brain injury or stroke.
The percentage of the population with developmental dyscalculia is estimated to be between 3-6 percent, or one in twenty individuals [2-4]. This is a similar percentage to that for dyslexia, and yet dyscalculia is very understudied and under-resourced in comparison. Unlike some other learning disabilities, dyscalculia is as likely to affect girls as boys.
 L. Kosc, "Developmental Dyscalculia," Journal of Learning Disabilities, vol. 7, pp. 164-77, 1974.
 V. Gross-Tsur, O. Manor, and R. S. Shalev, "Developmental dyscalculia: prevalence and demographic features.," Dev Med Child Neurol, vol. 38, pp. 25-33, 1996.
 N. A. Badian, "Persistent Arithmetic, Reading, or Arithmetic and Reading Disability," Annals of Dyslexia, vol. 49, pp. 45-70, 1999.
 C. Lewis, G. J. Hitch, and P. Walker, "The prevalence of specific arithmetic difficulties and specific reading difficulties in 9- to 10-year old boys and girls," Journal of Child Psychology and Psychiatry, vol. 35, pp. 283-292, 1994.