Can a poem have one word that is considered a line? https://www.poetryfoundation.org/poems-and-poets/poems/detail/47558 This 18 line poem, in the third stanza, the word tomorrow is by itself. can it still be considered as a line?

There are a number of lines in that particular poem that have a line to themselves. Personally I wouldn't refer to them as lines but as layout or format. (That's an uneducated comment. Poetry ain't my strong point.)

However, some standard poems using standard metric foot can accomplish a whole line in a single word.

The metre I'm thinking of is double dactyl. (Dactyl is a long followed by two shorts. A double dactyl does it twice.) In the following poem "incomprehensible" is a double dactyl and, consequently a line, all by itself.

Tiddly quiddly
Edward M. Kennedy
Quite inadvertently
Drove in a stream.

Pleas of amnesia
Incomprehensible
Probably shattered
Political dream.