In English numeric abbreviations and currency:
- K for thousand (from kilo)
- M for million
- B for billion
- T for trillion
In metric prefixes for SI measurements:
- k for thousand (note lowercase)
- M for million (from mega)
- G for billion (from giga)
- T for trillion (from tera)
Therefore, 25K is 25,000 units. You could also write 25k (lowercase k). These abbreviations are appropriate for summary text but not itemized lists or tables. In that case, should not abbreviate (although you may specify all numbers in thousands in the legend if appropriate).