Not even 'straight' is cut and dried
Sexuality is a spectrum, and there's more than one way of being straight. It's true that people who identify this way are in general attracted only to people of a different gender to them - but that can cover a lot of bases! There are more than two genders, after all; there are people who identify as straight who are attracted to others of any gender other than their own. Some straight people are attracted primarily to gender identity, and have relationships with trans partners, while other may find that they seem only to be attracted to cisgendered people. Not even 'straight' is a cut and dried sexuality - never forget that humans are complex, and so therefore are the words they use to describe themselves!
What does heterosexual mean?
Someone who is exclusively romantically and or sexually attracted to people of the opposite gender/sex. So a woman who is attracted to men or a man who is attracted to women.
Is heterosexual the same as straight?
Yes, just as homosexuality is often expressed as a person being gay, Heterosexuality is often referred to as being straight.
Is heterosexuality the opposite of homosexuality?
It’s not as simple as that. Both can be seen as the far ends of the spectrum of sexual orientation with bisexuality sitting in the middle.
Can a heterosexual person be attracted to a person of the same sex?
It is definitely possible! Someone who is mostly attracted to people of the opposite sex may find someone of the same sex attractive. Just because you’re attracted to one person of the same sex as you doesn’t mean you’re necessarily gay or bisexual. It is up to that person to define what their sexuality is. If they identify as straight and not bisexual that is up to them.