“Field” has numerous meanings, most of which deal with an area that is relatively open and spacious. When it comes to land, various sports and agriculture use the term as one of their basic concepts.
Fields usually lack any large objects that would impede the ability of people to move around, such as trees or buildings. As such, the definition provided in the question is highly appropriate to the word “field”.
While “field” is most likely the intended answer to the question, there are numerous synonyms that would apply. Examples include grassland, meadow, pasture, plot, enclosure, and others. However, these meanings tend to be more restrictive and not adhere to the definition completely.