Novelty detection is the identification of new or unknown data

Data

The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...

 or signals that a machine learning

Machine learning

Machine learning, a branch of artificial intelligence, is a scientific discipline concerned with the design and development of algorithms that allow computers to evolve behaviors based on empirical data, such as from sensor data or databases...

 system is not aware of during training. Novelty detection is one-class classification. The known data form one class, and a novelty-detection method tries to identify outlier

Outlier

In statistics, an outlier is an observation that is numerically distant from the rest of the data. Grubbs defined an outlier as: An outlying observation, or outlier, is one that appears to deviate markedly from other members of the sample in which it occurs....

s that differ from the distribution of ordinary data, which formed the single data class. Compared to multi-class classification, one-class classification

One-class classification

One-class classification tries to distinguish one class of objects from all other possible objects, by learning from a training set containing only the objects of that class. This is different from and more difficult than the traditional classification problem, which tries to distinguish between...

 is useful if outliers are sparse compared to ordinary data.