Billy Zane's hair in Titanic has become a tiny internet mystery because it looks almost too perfect: glossy, controlled, period-appropriate, and strangely modern at the same time.
The cautious answer: many beauty and fan discussions describe Cal's hair as wig-like or refer to it as a wig, but the strongest public sources are not a formal studio hair department document. So the fair way to say it is: Cal's look appears to have been created with theatrical hair styling, and widely circulated commentary treats it as a wig or wig-assisted look, but you should be careful about calling it an officially confirmed fact unless citing a production source.
Why people believe it was a wig
Cal's hair has the smoothness and silhouette of a controlled costume piece. In a period film, wigs are common because they protect continuity and help actors fit a specific era. A wealthy 1912 character like Cal would not have casual, messy hair. His look needed to read as money, control, vanity, and status before he even spoke.
What ranking pages usually say
Top search results are a mix: entertainment commentary, beauty retrospectives, and fan threads. They agree on one thing: the hair is a character detail. Whether it is a full wig, a piece, or heavy styling, the result helps Cal look severe and polished next to Jack's looser, romantic hair.
The better question
Instead of asking only "was it a wig?", ask what the hair does in the movie. It creates a visual wall between Cal and everyone around him. It is stiff where Jack is relaxed. It is expensive where third-class hair is windblown. That is why people remember it.
Bottom line
Billy Zane's Titanic hair is widely discussed as a wig or wig-like period look, but the most reliable answer is nuanced: it was a deliberately designed costume hairstyle, and the public evidence is stronger for "movie hair construction" than for one simple yes-or-no claim.