Abbreviation of 固定電話 (landline telephone). Common in spoken language or informal contexts (notes, business cards); formal texts typically use the full term.
名刺には固電と携帯の番号が両方載っている。
The business card has both a landline (固電) and a mobile number.
連絡先：固電03-1234-5678、携帯090-9876-5432
Contact: landline (固電) 03-1234-5678, mobile 090-9876-5432
固電という表現はやや口語的で、正式には固定電話と書くのが無難だ。
The term '固電' is somewhat colloquial; it's safer to write '固定電話' in formal contexts.