Was ist der Unterschied?

[worldly pasture]

Was ist der Unterschied zwischen "Spiegel" und "Spieglein"?🤔

[wintry pelican]

-lein is the diminutive suffix like -chen, so it basically indicates either small size or endearment.

If you're thinking of "Spieglein, Spieglein an der Wand..." then it is semantically close to "dear/precious mirror on the wall".

[grizzled fox]

Spiegel is Mirror