Trusted design is a term used in connection with Trezor security. It means that a Trezor device or its parts can be trusted in carrying out all the operation correctly under any realistic condition.
For example, information displayed on Trezor trusted screen can by relied on as being correct without any danger of it being modified or compromised by malicious software. Similarly, Trezor One's buttons and Trezor Model T's touchscreen are both trusted inputs as entered information cannot be modified or leaked in any way. Thus any information displayed or enter on Trezor can be trusted as being effectively true and private.