Is the term "halfcast" racist?

In BrE this term is definitely frowned upon. An acceptable analogue is "mixed-race".

The term 'half-caste' is indeed used to describe someone of mixed-race (mixed-parentage, mixed-heritage, or whatever other term is in vogue now). The reason that it is offensive however, is because the word itself has nothing to do with ethnicity.

Caste is a word describing social grouping or status and the term half-caste describes somebody who is not completely worthy of high caste status because one of their parents is from a lower caste, as was the case when black people were considered by the consensus to be of lower privilege than white people.

To still use this word now is to suggest that difference in social status still exists and thus its offensiveness is in part similar to the way 'the n-word' is offensive. Of course, a person who uses the term today might be wholly unaware of its original connotation but indeed it is clear why some might be offended by it.

In agreement with others, I would say this term is at the very least insensitive now, though it doesn't seem to be as inflammatory as, say, "coloured" in the States. (I'm not from the States, but have seen reactions to its use in the media, such as Lindsay Lohan's use of it to describe Obama in 2012).

There are many terms which were at some time simply descriptive (half-caste simply making a statement about caste; a while back retarded simply described a condition), but as societies evolve and values change, it can become unacceptable to identify people according to outdated classifications.

In my father's country, I would be called afakasi, which is a Samoan loan-word which of course stands for half-caste. It's a fluid term, its offensiveness depending on who is using it or why. I might describe myself to someone as afakasi to point out that I am part Samoan, part something else. In Samoa, some pure bred Samoans will call someone afakasi in a pejorative sense to put them down.

While not the most offensive of racial terms, it certainly won't hurt us for the term to one day slip into the "totally unnecessary and unacceptable" folder.

I have used the term myself and certainly never meant it in a derogatory way, sometimes describing my own family.

Wikipedia states:

Half-caste is a term for a category of people of mixed race or ethnicity. It is derived from the term caste, which comes from the Latin castus, meaning pure, and the derivative Portuguese and Spanish casta, meaning race. It can sometimes be used in an offensive manner but not universally (particularly in New Zealand and the Pacific Islands and parts of Asia).

I shall have to find the current 'acceptable term'...

As a mixed person which is a voice I find strangely lacking from these answers that indeed half cast is offensive. Honestly a mixed person telling you that a word about them affects is offensive to them should be the end of the argument. Like if a black person finds the n word offensive you don't use it end of story.

I find this double standard a lot. This word has never been and never will be okay. So just like if any other race tells you not to call them that just don't.