adverb
Indicates a constant state or habitual action, similar to 'always' or 'at all times'.
彼はいつでも親切だ。
He is always kind.
この店はいつでも混んでいる。
This shop is always crowded.
adverb
anytime; whenever
Used for offering or stating that something is possible at any time, or whenever one likes.
いつでも好きなときに来てください。
Please come whenever you like.
質問があればいつでも聞いてください。
If you have questions, please ask anytime.
いつも also means 'always' but is more common for habitual actions. いつでも emphasizes 'at any time' or 'whenever', while いつも is closer to 'every time' or 'usually'.
常に is a more formal/literary word for 'always', often used in written language or formal speech, whereas いつでも is neutral and conversational.
Compound of いつ (when) + でも (even, also). The combination yields the meaning 'at any time' or 'always'.