Alien can be boiled down to simply meaning "foreign". As in, not coming from the wire bundle that is used for the connection.

Crosstalk is the term usually used for interference coming from other pairs in the same bundle, which should be minimal due to the electrical/magnetic properties associated with twisted pair.

Alien interference is any inference from an outside source, usually by inducing a current on the Ethernet pairs, that shouldn't be present. Usually this results in corruption of the data in transit or a failure to sync (and establish a connection at all).

No extra terrestrial interference was meant to be implied; though, I'm not excluding the extra terrestrials from creating interference; I'm sure if such beings exist and are here, they could interfere, but that wasn't the intention of my statement.

Alien inference is a very common term in wireline networking. I'm surprised you haven't heard it.

Alien meaning "external".

Electrical interference can come from all kinds of places, near and far. I guess technically you might get interference from other planets but I don't think that's what they meant. :) Solar flares are a possibility, though.