An arachnid but not a spider.
Daddy, how can it be an arachnid but not a spider?
Well you see, Son, all spiders are arachnids but not all arachnids are spiders.
Oh. What's the difference?
There is no difference. Arachnids are a group that includes spiders but also includes other things.
Oh. But they look like spiders.
Yes they do.
Then why aren't they spiders?
Their eight legs make them look like spiders, but these have a fused body region and only one pair of eyes. Spiders have a jointed body and a bunch of eyes.
Oh. Why be so particular about it?
That's how scientists are. They group things specifically so they can understand them.