Harassment is where someone’s dignity is violated, or they suffer an intimidating, hostile, degrading, humiliating or offensive environment. Harassment on the basis of a protected characteristic is prohibited by the Equality Act 2010 - and if the harassment is of a sexual nature, it will amount to sexual harassment.
For example, if a colleague persists in making remarks about what nice legs a female employee has, or her boss promises her promotion if she goes away with him for the weekend, she should be able to claim that this is sexual harassment and ask her employer to do something about it.
