Before discussing why Shiva is called Ardhanarishwar, let us first split up this word and understand its meaning. (Ardha – half, Nari – woman, Ishwar – Lord) The meaning is the Lord who has the feminine form in half of his body. As per this concept, Lord Shiva is shown in his androgynous form with Mother Parvati, his consort sharing half of his body in the left side. Since the Mother is on the left side, the Lord is also praised as Parvati Vama Bhagam. Here the word Vama means left and Bhagam means part. Here the description means The Lord has given away his left part to Mother Parvati.
Going to the symbolism behind the concept of Ardhanarishwar, we can draw parallels to the concept of matter and energy and positive and negative (Proton and electron). The union of male and female energies is at the base of all creation. Therefore Shiva and Shakti together form this universe. Also, the depiction brings to surface the equal position granted to women in Indian culture – man and woman are made equal and are hence considered equal. We are made to realize that Shiva and Shakti are inseparable.
A Sanskrit verse composed in praise of Lord Shiva and Parvati heralds this concept of Shiva and Parvati being the Parents of this Universe.
वागर्थाविव संपृक्तौ वागर्थप्रतिपत्तये।
जगतः पितरौ वन्दे पार्वतीपरमेश्वरौ॥ १-१
vāgarthāviva saṁpṛktau vāgarthapratipattaye |
jagataḥ pitarau vande pārvatīparameśvarau || 1-1
As the word and its meaning are inseparable, The parents of the universe Parvati and Parameshwara exist as a two-in-one entity and I bow down to them in reverence.
Now we move on to read the mythological story behind the Ardhanarishwar concept. Once upon a time, Lord Shiva and Parvati were spending time in a beautiful garden on the Himalayas. In a playful mood, Mother Parvati came from behind and cupped her hands to close the eyes of Shiva. Though it was a momentary sport for them, it is said a blinding darkness engulfed the whole creation for millions of years. Mother Parvati realized her mistake and asked for the Lord’s forgiveness. The Lord advised her to undertake a strict life of penance and austerities on the earth to absolve the sin she had incurred.
Mother Parvati descended on the earth first as Annapurni is Kashi, then as Kamakshi in Kancheepuram and then Akilandeswari in Tiruvannamalai. In all these three places, the divine Mother spent her time in a severe penance and austerities. She also did a lot of charity to all the devotees who came to her. Lord Shiva subjected her to several tests, trials, and tribulations which the Mother passed with her diligence and devotion. Finally, Lord Shiva asked her to seek a boon from Him. Mother Parvati requested half of his body so that she could always be united with him. Owing to her prayers, Lord Shiva granted the left side of his body to Mother and the form Ardhanarishwar came into existence. Therefore, Ardhanrishwar is the fusion of Prakruti (Parvati or matter) and Purusha (Shiva or energy)or the masculine and feminine energies of the universe.