For information (from http://www.informatics.sussex.ac.uk/department/docs/punctuation/node28.html )
There are a number of Latin abbreviations which are sometimes used in English texts. Here are the commonest ones with their English equivalents:
e.g. for example cf. compare i.e. in other words v. consult viz. namely etc. and so forth sc. which means et al. and other people ca. approximately
The rule about using these Latin abbreviations is very simple: don't use them. Their use is only appropriate in special circumstances in which brevity is at a premium, such as in footnotes. It is very poor style to spatter your page with these things, and it could be disastrous to use them without being quite sure what they mean. If you do use one, make sure you punctuate it correctly. Here is an example. The recommended form is this: Several British universities were founded in the Victorian era; for example, the University of Manchester was established in 1851. The following version is not wrong, but it is poor style: Several British universities were founded in the Victorian era; e.g., the University of Manchester was established in 1851. But this next version is disastrously wrong, because the punctuation has been omitted: *Several British universities were founded in the Victorian era e.g. the University of Manchester was established in 1851. Using a Latin abbreviation does not relieve you of the obligation of punctuating your sentence. Again, if you avoid Latin abbreviations, you won't get into this sort of trouble.
The abbreviation ca. `approximately' is properly used only in citing a date which is not known exactly, and then usually only if the date is given in parentheses: The famous Basque cemetery of Argiñeta in Elorrio (ca. ad 883) shows tombs with sun-discs but no crosses.
Roger Bacon (ca. 12141294) was known as "the Admirable Doctor".
Here the use of ca. shows that the date of the cemetery and the date of Bacon's birth are not known exactly. If neither birth date nor death date is known for sure, then each is preceded by ca.
Outside of parentheses, you should usually avoid the use of ca. and prefer an English word like about or approximately:
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