An excerpt from the novel says "Editor’s note: The human concept of friend is most nearly duplicated in Vulcan thought by the term t’hy’la, which can also mean brother and lover."
T'hy'la is popularly used in Kirk/Spock slash stories, where "soul-mate" and "life-long companion" are often accepted within fanon as additional definitions of T'hy'la.
-- Spock's thoughts before almost undergoing Kolinahr, a Vulcan ritual to purge all emotion. (Star Trek: The Motion Picture novel, page 14-15.)
There was much to be put out of his mind. Why was it difficult to forget Chekov’s astonished delight which greeted him at the command airlock when he boarded? And on the bridge—Kirk! The mere name made Spock groan inwardly as he remembered what it had cost him to turn away from that welcome. T'hy'la!
-- (Star Trek: The Motion Picture novel, page 109.)