restlich
remaining
What is left of something after a part has been used up, removed, or completed.
Den restlichen Kuchen können wir morgen essen.
We can eat the remaining cake tomorrow.

remaining
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remaining
What is left of something after a part has been used up, removed, or completed.
Den restlichen Kuchen können wir morgen essen.
We can eat the remaining cake tomorrow.

The adjective 'restlich' (remaining) is derived directly from the noun 'der Rest' (the rest, the remainder). If you know 'der Rest', you can easily figure out the meaning of 'restlich'.
Den Rest des Tages möchte ich mich nur noch entspannen.
For the rest of the day, I just want to relax.
'Restlich' (remaining) is used almost exclusively before a noun (attributive use). Using it as a stand-alone adjective after a verb (predicative use) is very uncommon and sounds unnatural.
Die restlichen Studenten warten schon im Seminarraum.
The remaining students are already waiting in the seminar room.
Common everyday combinations are 'der restliche Tag' (the rest of the day), 'das restliche Geld' (the remaining money), and 'die restlichen Aufgaben' (the remaining tasks). It always describes the part that is still left.
Mit dem restlichen Geld können wir uns noch ein Eis kaufen.
With the remaining money, we can still buy ourselves an ice cream.
left over
'übrig' and 'restlich' both refer to something that remains after a part is gone. 'übrig' is used very often with 'sein' (to be), 'bleiben' (to remain), or in sentences like 'Es ist noch etwas übrig' (There is still something left over). It sounds everyday and can stand on its own. 'restlich' usually comes before a noun, for example 'die restliche Zeit' (the remaining time) or 'das restliche Geld' (the remaining money). It describes the remaining part of a larger amount. The words can be confused because both refer to a remainder. In practical terms: 'übrig' works well as a complement in a sentence, while 'restlich' works well directly before a noun.

Sie hat das restliche Geld für die Fahrkarte gespart.
She saved the remaining money for the ticket.
Beim Einkaufen sind noch zehn Euro übrig.
After shopping, there are still ten euros left over.