Definition
Self-transcendence is a basic feature of human existence: a person is always directed towards something or someone beyond themselves - towards a meaning to fulfil or another person to meet and love.
The eye that does not see itself
Frankl used the comparison of the eye: a healthy eye does not see itself. When it starts “seeing itself” (e.g. clouding in a cataract), it is a sign of illness. Similarly, a person fulfils themselves only by forgetting themselves - by giving themselves to a cause or a person.
Self-actualisation as a by-product
Frankl criticised making self-actualisation an end in itself. Self-actualisation - like happiness - comes as a by-product of self-transcendence. Whoever aims at it directly misses it.
In therapy
The technique of dereflection is based on this principle - redirecting attention away from oneself and one's symptoms towards tasks and people.
Sources
V. E. Frankl, The Will to Meaning; Man's Search for Meaning (postscript).