IPA: /θɪəˈrɛtɪkli/
KK: /θiˈɔrɪkli/
In a way that is based on theory rather than practical application or experience.
The project is theoretically sound, but we need to test it in real-world conditions.
The word 'theoretically' is formed from 'theoria' (from Greek 'theoria', meaning contemplation or speculation) and the suffix '-ically' (meaning in a manner relating to). Thus, 'theoretically' refers to something considered in a speculative or abstract manner, rather than practically.
Think of 'theoria' as contemplation or speculation, which helps you remember that 'theoretically' means considering something in an abstract way.