The principle of consistency enables people to learn new systems faster and efficiently deliver knowledge to new frameworks. The systems are more usable and learnable when they are presented in similar ways. Consistency helps people focus their attention on the task at hand. There are four categories of consistency: aesthetic, functional, internal, and external.
Aesthetic consistency means consistency in style. Aesthetic consistency helps things easily identifiable for people. As an example, a company logo is great identifier of who the company is and what the company does. When the logo is used consistently (same font, color, graphic, and tagline), then a brand is born. Aesthetic consistency enhances recognition and communicates emotional connection between brand and people.
Functional consistency improves usability by formulating existing knowledge of how designs functions. When similar controls function the same way, it is called functional consistency. The consistent use of symbols on a new device increases predictability of the product, which makes the new device easier to use and learn.
Internal consistency relates to correlation of elements in the system. This is a blend of both visual and functional consistency. As an example, if updating a webpage with new elements, you should also update other older webpages in the website with same elements.
External consistency refers to consistency across multiple independent systems. Adobe products are good examples of external consistency. Adobe Photoshop and Adobe Illustrator have similar tools and features that is recognizable to the users. Therefore, if you know Photoshop you can apply that knowledge to learn Adobe Illustrator.
Visual depiction from Universal Principles of Design

“Bob Evans uses the same logo, typefaces, color, schemes, menus, staff uniforms, interior design, and architecture across its restaurants. The consistency improves brand recognition, reduces costs, and establishes a relationship with customers that extends beyond any single restaurant.” (Lidwell, Holden, Butler 2010, p.57)
Online visual depiction

Elements of website designs to include Aesthetic consistency, Functional consistency, Internal consistency, (https://gofishdigital.com/guide-design-consistency/) and External consistency. (https://uxdesign.cc/design-principle-consistency-6b0cf7e7339f)
Real life visual depiction

Aesthetic consistency: Apple logo on MackBook Pro, iMac, Magic Mouse in Silver and in Space Gray

Functional consistency: Roku remote control, Samsung LED remote control, Samsung Blue-ray remote control (play, pause, fast-forward and rewind buttons)

Internal consistency: In the city of Irvine, street signs are brown with white letters

External consistency: Ubiquitous and recognizable restroom signs
References:
William Lidwell, Kritina Holden, Jill Butler (2010), Universal Principles of Design, Beverly, MA: Rockport, 2010
Matt Burt (2017) A Beginner’s Guide to Achieving Web Design Consistency, Go Fish Digital, https://gofishdigital.com/guide-design-consistency/
Anton Nikolov (2017) Design principle: Consistency, The most known and the most fragile design principle., UX Collective, https://uxdesign.cc/design-principle-consistency-6b0cf7e7339f