Though "conquest" does seem an appropriate term for the encounters between the Spanish and Natives in the 16th century, Restall points out several factors that call the term's applicability into question. On the face of it, Spanish conquistadors did bring down Maya, Nahuatl, Inca and others' leaders and install colonial or puppet leaders (such as replacing Moctezuma with Cuauhtemoc). This is typically defined as conquest but Restall cites three main factors (while adequately dismissing others) as giving the Spanish the edge necessary to do this that make the term "conquest" problematic. One was Spanish war methods that ranged from steel weapons to a culture accustomed to total war rather than ritualized war methods (143-4).
If this were the only factor perhaps "conquest" would be appropriate but Native disunity and European pathogens provided the Spanish with their most valuable military assets (140-1). Once in contact with Native groups the Spanish could use rivalries and resentment between the localized polities of Mixtecs, Zapotecs, Maya (a vastly diverse ethnic and cultural group in itself that sports, to this day, over 40 language groups), Mexicas, Tlaxcalans et al. Divide and conquer was hardly a foreign strategy to the Europeans but it is just as likely that the Natives were using the Spanish to advance their own political clout.
Easily the biggest asset to the Spanish however, and the most compromising to the term "conquest" are the microbial shock troops: Smallpox, Measles, Plague, and Influenza, to name a few. These swept ahead of any expedition and wiped out native populations, killing both potential fighters and morale among those left. The Spanish were not even fully aware of why the Native population was in such precipitous decline in the first decades of colonial encroachment (128).
Conquest is a problematic term for the encounter between the Ameican Natives and Spanish in the late 15th and early 16th centuries. It was was more of a colonial collision with the Europeans blundering into a completely foreign world and making ad hoc allies and enemies to compete for eceonomic viability in a land rich in resources.