If you describe something as ~, you mean that it is, for example, extremely beautiful, enjoyable, or unusual, so that you remember it for a long time. You can also refer to extremely unpleasant things as ~.
A visit to the Museum is an ~ experience...
...the leisure activities that will make your holiday ~.
ADJ
unforgettably
...an unforgettably unique performer.
ADV: usu ADV cl/group