Can you help me define Allusion?

If you can also give me an example and find what are the illusion in this poem: what god has promised

God has not promised skies always blue,
flower-strewn pathways all our lives through;
God has not promised sun without rain.
joy without sorrow, peace without pain.
But God has promised strength for the day,
rest for the weary, light for the way,
grace for the trials, help from above,
unfailing sympathy, undying love.

Question

Can you help me define Allusion?

If you can also give me an example and find what are the illusion in this poem: what god has promised

 God has not promised skies always blue,
 flower-strewn pathways all our lives through;
 God has not promised sun without rain.
 joy without sorrow, peace without pain.
 But God has promised strength for the day,
 rest for the weary, light for the way,
 grace for the trials, help from above,
 unfailing sympathy, undying love.

anonymous · Answer

allusion-An expression designed to call something to mind without mentioning it explicitly; an indirect or passing reference.