Approx is a common abbreviation used to indicate an approximate or estimated value or quantity. However, there are several synonyms that can be used in its place, depending on the context of the sentence. Some of the common synonyms include 'roughly,' 'about,' 'more or less,' 'around,' 'approximately,' 'nearly,' 'almost,' 'virtually,' 'in the ballpark of,' 'close to,' and 'just about.' Each of these synonyms can be used interchangeably with approx in sentences like 'The cost of the project is approx $50,000,' or 'Approx 80% of the population is vaccinated.' It's important to choose the appropriate synonym based on the context since synonyms can have different nuances.