In the US what is/are the legal definitions of 'workplace'?
Absent a statutory or contractual definition, the plain meaning is adopted "unless doing so would result in absurd, unintended consequences", Hassell v. Bird, 5 Cal.5th 522 (2018).
Pulaski v. California OSHA, 90 CalRptr.2d 54, 69 (1999) points out that "'[w]orkplace' is commonly understood as covering any place where work is performed. This is especially true where worker health and safety is concerned". See also [non-precedential] [Covia Communities v. McInerney, (Court of Appeals of California, Dec. 2019)]3 ("The plain meaning of the term [...] does not limit 'workplace' to one location [...]. Nor does the plain meaning of 'workplace' require that the employer own the subject property").
Thus, a person's homeoffice also fits the definition of 'workplace'.
Can an employer dismiss a smoker for smoking at home during mandatory homeoffice (and on a teleconference) or would this be perceived as discriminatory?
Discriminatory means that the dismissal is motivated by the status of employee being insofar as a smoker rather than his act of smoking during the performance of work. By way of analogy, consider the laws against discrimination based on a person's sexual orientation: The ban on that discrimination does not entitle the employee to engage in sexual intercourse during a teleconference.
Most bans on smoking in the workplace are intended to protect other employees' health and safety. From the remark in Pulaski, it follows that smoking at home during mandatory homeoffice time would not impair other employees' health and safety.
That being said, the employer might have valid reasons for prohibiting to smoke during a teleconference. For instance, doing so could be perceived as unprofessional, and therefore detrimental to the image of the company.