The term CA commonly means:

• California, a U.S. state
• Canada, a nation on the North American continent
• Chartered Accountant
• Compressed Air, such as "CA Piping"
• Certificate Authority
• Cyanoacrylate, a chemical compound

See numerous other uses below, by category.

The term ca commonly means (more below):

• circa, as meaning approximately (ca.)
• .ca, Internet country code for Canada

The term Ca commonly means:

• Calcium, as a symbol for the chemical element (Ca)

The term cA commonly means:

• Centiampere, an SI unit of electric current (cA)

.ca is the Internet country code top-level domain (ccTLD) for Canada. The domain name registry that operates it is the Canadian Internet Registration Authority (CIRA). Registrants of .ca domains must meet the Canadian Presence Requirements as defined by the registry. Examples of valid entities include:

• a Canadian citizen of the age of majority
• a permanent resident of Canada
• a legally recognized Canadian organization
• an Inuit, First Nation, Métis or other people indigenous to Canada
• an Indian Band as defined in the Indian Act of Canada
• a foreign resident of Canada that holds a registered Canadian trademark
• an executor, administrator or other legal representative of a person or organization that meets the requirements
• a division of the government
• Elizabeth II in her capacity as head of state of Canada

Calcium (Ca²⁺) plays a pivotal role in the physiology and biochemistry of organisms and the cell. It plays an important role in signal transduction pathways, where it acts as a second messenger, in neurotransmitter release from neurons, contraction of all muscle cell types, and fertilization. Many enzymes require calcium ions as a cofactor, those of the blood-clotting cascade being notable examples. Extracellular calcium is also important for maintaining the potential difference across excitable cell membranes, as well as proper bone formation.

Circa , usually abbreviated c. or ca. (also circ. or cca.), means "approximately" in the English language, usually referring to a date. Circa is widely used in genealogy and historical writing when the dates of events are not precisely known.

Circa is a Latin word meaning "about" or "around".