さき (saki) is the kun-yomi used when the kanji stands alone meaning "future", "ahead", or "before".
セン (sen) is the on-yomi used in compounds like 先月 (せんげつ, sengetsu: last month) and 先輩 (せんぱい, senpai: senior).
Etymology: Stop (止) + person (儿) → person who lived before → previous, ancestor
Note: 先 can mean both "previous" (in time) and "ahead" (in space), depending on context. In compounds like 先月, it means "previous month", but when used alone as さき, it often means "ahead" or "future".
Examples
Kun-yomiNative Japanese Readings
先へいきます。
I go ahead.
Uses kun-yomi さき (saki) when the kanji stands alone meaning future or ahead
On-yomiSino-Japanese Readings
先月です。
It's last month.
Uses on-yomi セン (sen) in compound 先月
先輩です。
It's a senior/mentor.
Uses on-yomi セン (sen) in compound 先輩
Quick facts
Radical: 儿 (person)
Strokes: 6
Common compounds: 先月 (せんげつ, sengetsu), 先輩 (せんぱい, senpai)
⚠️ Reading usage:
Standalone: さき (saki) - future, ahead
In compounds: せん (sen) - previous, before (time context)