The MNIST database (Mixed National Institute of Standards and Technology database) is a large database of handwritten digits that is commonly used for training various image processing systems. The database is also widely used for training and testing in the field of machine learning. It was created by "re-mixing" the samples from NIST's original datasets. The creators felt that since NIST's training dataset was taken from American Census Bureau employees, while the testing dataset was taken from American high school students, NIST's complete dataset was too hard.
MNIST (Mixed National Institute of Standards and Technology database) database
Handwritten digit database
$28 \times 28$ gray scaled image
Flattened matrix into a vector of $28 \times 28 = 784$