Why is every TV show's first episode named 'Pilot'?

it's not some shows...it's ALL shows (or all fictitious shows at least). The pilot is the episode they film to get approval from the network whether they're going to be signed or whatever. Not all pilots even air (sometimes not even the ones of successful shows). Sometimes they'll re-shoot the first episode, maybe even re-write it because the pilot is the "test". As to why they call it a pilot, I'm not exactly sure...it's probably a metaphor for a maiden voyage or something.