I'm a scientist, so this might sound like a stupid question to people who are experts in disciplines where 'gender' is used, but I'd appreciate an explanation of what 'gender' means in this context and why it is used rather than any other term with a similar meaning
Well, first, I'm not sure that any discipline needs to 'justify itself' to anybody.
Second, see my post above about 'sex roles'. These used to refer to gestating and birthing, or ejaculating and snoring, and so on.
The debate used to be about how much 'sex roles' were about 'gender or nature' - for example, about biology or social conditioning. It was, and is, a given that women gestate and birth and also can choose to breastfeed. These are about 'sex roles'. However, whether we choose to stay home and be SAHM and so on is about 'gender' - this is the socially constructed element and conservatives used to argue that this was 'nature' and we used to argue this was 'nurture' (social conditioning).
Gener gave us a handy term to separate the two.
Please do bear in mind that I m referring to how I learned about feminism from my slightly older second-wave sisters when I was in my 20s in the 80s. I am talking about history, my experience, and my discipline (in which I have a Ph.D.). This was important and meaningful to us and it remains so to me.
Gender, used carefully and in particular ways, is useful to designate something that 'isn't sex' (biology) and which is about the power of men to define who women are socially.
It remains, to me, a useful term.
Now, if anyone wants to challenge that, then I suggest that they actually read the seminal texts of women's studies, which go beyond Greer and Dworkin and include Baker-Miller, Rowbotham, Oakley, Mitchell, Firestone, Eistenstein, Millett and so on.
Having said that, there was some argument in the late 80s that 'sex and gender' were the same thing in ontological terms. I suggest you Google or use your university library site to find Denise Thompson for this.