Is sexual fantasy about someone other than one's spouse during marital relations or in the early days of marriage considered actual Zina (fornication/adultery) that necessitates the Hadd punishment, or is it a metaphorical Zina?

For a woman to imagine another man during intercourse with her husband is not permissible. However, it is not considered zina (fornication/adultery) that necessitates a prescribed punishment (hadd). Its expiation is repentance to Allah, by ceasing the sin, regretting it, and resolving not to return to it. One should not succumb to the whispers of Satan, but rather occupy one's time with what benefits one in matters of religion and worldly life, and cooperate with the husband in obedience to Allah and frequent remembrance of Him.