Answer:
Ticks are little critters that eat at human blood, but humans do not actually get harmed by it. Yes, one could get transmitted diseases from the tick, but the tick itself is not harming the human.
A parasite, on the other hand, gets their food and benefits AT THE EXPENSE of their host. This means that the parasite causes harm directly to the host.
The tick does not cause direct harm, so it is more like a commensal, not like a parasite.