. A harmonious uniformity or agreement among things or parts is called consistency.
. It is a kind of repeated actions, all having same characteristics.

Types of consistencies:

. Visual Consistency: 
    . Elements should look in similar manner. 
    . Font Styles, Sizes, Colors. 
    . It increases learnability of product.

. Functional Consistency: 
    . Functionality of elements should be consistent.
    . Usage of symbols for a purpose should be same all over the application.
    . It increases predictability.

. Internal Consistency: 
    . Combination of visual and functional consistency.
    . It improves usability and learnability of product.

How to make a design consistent?

. Always use appropriate colors to elements of same type.
. Use consistent spacing, size and positioning between the elements.
. All icons should be of same family.

Few things to keep in mind in order to create consistency:

. Color scheme
. Borders
. Font size and type
. Background images
. Effects and styles

Outcomes of consistent designs:

. Gives sense of identity
. Ease of navigation
. Good communication of information