The word Muslimah is a perfectly legitimate Arabic word. It is the
feminine form of the word Muslim and used thus very commonly. However, if
someone uses it only because he or she thinks that the word Muslim connotes the
male gender only, then this would be an incorrect inference. The word Muslim
covers both men and women. Consequently, you will often hear the sentence ‘She
is a good Muslim’. There are several other words which are used irrespective of
gender, for example the words ‘writer’ and ‘poet’.
It seems that it is because of the influence of the feminist
movement, which started a couple of centuries ago, that the feminine gender of
words used for both genders have started springing up. The word ‘chairman’ and
‘webmaster’ are irrespective of gender. But now the trend is to ‘feminize’ them
as well. Thus you now have a chairwoman and a webmistress.