This elective course is very popular and many students have signed up.

elective 是由动词 elect（选择）加上后缀 -ive 构成的形容词，表示"可选择的"或"非强制性的"。 course 的意思是"课程"，指学校开设的教学科目。 elective course 指学生可以根据个人兴趣自由选择的课程，即"选修课"，与必修课(required course)相对。