A Fascist is someone who ideologically supports either the creation or the perpetuation of a Fascist regime. Fascist regimes are not all alike, but all have certain common characteristics: authoritarian leadership (most often under a single dictator figure, but really any non-representative government that violently suppresses domestic opposition and severely regiments its population ticks this box), nationalism (the belief that one's nation is superior to all others), and exploitation of that nationalism to create an enemy within the population (e.g. Jews in Nazi Germany).
The US' path toward Fascism is clear, but the likelihood of it actually occurring is almost nonexistent. Trump would have to abolish term limits for himself, either falsify election results to the point of creating a 100% Republican Congress or do away with elections altogether, and vilify a group (most likely people of Hispanic descent, though there's wiggle room as opportunity arises) to the point of state-sanctioned violence against them. It's not impossible, but it's certainly implausible that it would get that far at all, and nearly impossible that it would happen without a civil war.