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from a time in the past until now
She's been working here since last year.
This means she started working last year and continues to work now.
Often used with perfect tenses (have/has been) to indicate an action that started in the past and continues.
because
Since you're here, let's have coffee.
This means 'because you're here, let's have coffee.'
This meaning is more common in informal contexts.
from the time when
Since the weather is nice, let's go outside.
This means 'from the time when the weather is nice, let's go outside.'
This meaning is less common and often interchangeable with 'because.'
Use 'since' to refer to time when it follows a time expression (e.g., 'since 2010'). Use it to refer to reason when it follows a clause (e.g., 'since you asked').
'Since' is often used with perfect tenses (have/has been) to indicate an action that started in the past and continues to the present.
From Middle English 'sins', from Old English 'sīðþan', from Proto-Germanic '*sīþan' (after, since).
The word 'since' can function as a conjunction, preposition, or adverb. Its meaning depends on the context and the tense of the verb it accompanies.