Terminology
The Lingual Technique is an orthodontic treatment method that involves fixing brackets on the inside of the teeth (i.e. on the ‘tongue side’). The Lingual Technique is the only real ‘invisible’ kind of orthodontic treatment as the brackets on the inside of the teeth are practically invisible to other people.
Aligner therapy, also marketed as invisible treatment, is not truly invisible, as you can very well see that another person is wearing aligners if you stand a short distance away and in good light. The lingual technique was developed because many people want to improve the position of their teeth, but are not willing to wear braces that are visible to other people.