Cross-training

Cross-training (also known as conditioning) refers to training in different ways to improve overall performance. It takes advantage of the particular effectiveness of each training method, while at the same time attempting to neglect the shortcomings of that method by combining it with other methods that address its weaknesses.

In general sports
Cross-training in sports and fitness refers to the combining of exercises to work various parts of the body. Often one particular activity works certain muscle groups, but not others, cross-training aims to eliminate this.

[edit] In Mixed Martial Arts
In mixed martial arts and self-defense applications, cross-training refers to training in multiple martial arts or fighting systems to become proficient in all the phases of unarmed combat. This training is meant to overcome the shortcomings of one style by practicing another style which is strong in the appropriate area. A typical combination involves a striking-based art such as Muay Thai, combined with a grappling-based art such as Wrestling and Brazilian Jiu-Jitsu. Many hybrid martial arts can be considered derivatives of such cross-training.

Modern mixed martial arts training generally involves cross-training in the different aspects and ranges of fighting.