UNDEFINED definition: 1. not clearly described, stated, or known: 2. not having been given a definition (= a statement…. Learn more. The meaning of undefined with an overview, related terms, and examples from programming. Understand how undefined variables can lead to common error messages. The meaning of UNDEFINED is not defined. How to use undefined in a sentence. This video explains the key differences between null and undefined in JavaScript, two concepts that often confuse beginners - we’ll break down what each one means, how they are used, and common. UNDEFINED meaning: 1. not clearly described, stated, or known: 2. not having been given a definition (= a statement…. Learn more. This disambiguation page lists articles associated with the title Undefined. If an internal link incorrectly led you here, you may wish to change the link to point directly to the intended article. If it is undefined, it will not be equal to a string that contains the characters undefined , as the string is not undefined. You can check the type of the variable: Share your videos with friends, family, and the world If something is undefined it's not clear, such as your undefined vacation plans. You know you want to go somewhere during the third week of August, but you just don't know where. You can tell just by looking at it that undefined means not defined. 1. not defined or explained. 2. indefinite in form or extent: an undefined feeling of sadness.