Correct spelling


Correct spelling, explanation: outer is the correct spelling because the base word out consists of two vowels and a consonant. In such case, when adding a suffix, there is no need to double the last consonant. Therefore the word should be spelled outer, not outter.

Definition of outer:
adjective, external, situated towards the outside
The outer part of the device is made of the waterproof material.
This product cleans outer surfaces very well.

Incorrect spelling


Incorrect spelling, explanation: outter is an incorrect form, and outer is correct, because there is no consonant-vowel-consonant combination at the end of the base word out. In such case we double the last consonant, but it’s not the case with word out. Therefore the correct form is outer – without doubled t.