My first semester of RN school we had a quiz on the most common abbreviations. Things such as lpm, TENS, c ("with"), s ("without"), and some others I don't remember
As others have said, each facility defines approved abbreviations and it is important to stick with those abbreviations and use them correctly. For instance, at one place I worked Rt. (with the capital R, lowercase t, and the period) was "right." RT (capital R and T, no punctuation) was "respiratory therapy." Now I believe it's fairly unlikely that those would be confused if you were reading charting where the wrong abbreviation is used...but, hey, you never know. It would mean big trouble for you if a pt. was harmed based on your use of a non-approved abbreviation. Point is, stick to the abbreviations approved by your facility.