Annulment refers only to making a voidable marriage null under section 12 of Hindu Marriage Act

Section 12 in The Hindu Marriage Act, 1955 [extract]

(1) Any marriage solemnized, whether before or after the commencement of this Act, shall be voidable and may be annulled by a decree of nullity on any of the following grounds, namely:-

(a) that the marriage has not been consummated owing to the impotency of the respondent; or

(b) that the marriage is in contravention of the condition specified in clause (ii) of Section 5; or

(c) that the consent of the petitioner, or where the consent of the guardian in marriage of the petitioner was required under Section 5 as it stood immediately before the commencement of the Child Marriage Restraint (Amendment) Act, 1978, the consent of such guardian was obtained by force or by fraud as to the nature of the ceremony or as to any material fact or circumstance concerning the respondent; or

(d) that the respondent was at the time of the marriage pregnant by some person other than the petitioner.

An annulment may be granted when a marriage is automatically void under the law for public policy reasons or voidable by one party when certain requisite elements of the marriage contract were not present at the time of the marriage