What was a land payment that the Spanish government gave to conquistadors.

The word that you are looking for is encomienda.  The
system of encomiendas fits pretty well with the definition that you have given
here.

In the encomienda system, the Spanish monarchy
rewarded the conquistadors for conquering new lands for Spain.  What they did was to
give the conquistadors the rights to the labor of a given number of Indians. 
Technically, the encomenderos (the men who were given the encomienda) did not actually
own the land.  The land technically still belonged to the Indians.  But the encomenderos
were given the right to force the Indians to work for them or to pay them tribute in
others forms.

So this was not technically a grant of land,
but the effect (while the system lasted) was much the same as if it were an outright
grant of land.