Question

1. spend:
to use money to buy or pay for something

2. spend:
use money to buy or pay for something
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word:
definition

(Why is 'to...' used in a dictionary for the definition of each word? What about #2? Can we use as in #2 as the definition of each word?)

Answer 1

I think because it doesn't make sense to say, "Spend is use money..." It does make sense to say, or write, "Spend is to use money..." It could also be properly (sensibly) phrased, "Spend is the use of money to buy or pay for something." Without the prepositions, neither would make sense.