usually; ordinarily; at all times
Adverbial use meaning that something is normally or consistently the case. It is somewhat more formal or written than everyday いつも.
This shop usually stocks more than twenty kinds of bread.
Security guards patrol at all times.
noun which may take the genitive case particle 'no'
continuous; 24-hour; always on
Used before another noun or in compounds for systems, operation, care, connection, monitoring, and similar states that continue without interruption.
An always-on internet connection is required.
That patient needs constant care.
The machinery in this factory operates around the clock.
A staff member is always stationed at the reception desk.
The security guard carries a radio with him at all times.
常に is a general literary adverb for 'always/constantly', whereas 常時 more specifically emphasizes a maintained state, availability, or continuous operation.
随時 in its 'at any time' sense implies continuous availability similar to 常時, but 常時 more strongly emphasizes an unbroken, constant state rather than on-demand access.
常時 is a stiff, technical word for 'at all times' (e.g. in manuals or notices), while いつでも is the everyday spoken equivalent.
非常時 (an emergency) is easily confused with 常時 (all the time) because of the shared kanji, but they mean nearly opposite things.
A Sino-Japanese compound of 常, associated with regularity or constancy, and 時, 'time'; literally, roughly 'constant times' or 'ordinary times.'