Short answer
Bid'ah means an innovation in religion: a new belief or act of worship, presented as part of Islam, with no basis in the Quran, Sunnah or the principles drawn from them. The Prophet ﷺ warned that every such innovation is misguidance. New things in worldly life, like cars and phones, are not bid'ah in this sense.
Hanafi texts define blameworthy bid'ah as holding or doing something contrary to what is known from the Prophet ﷺ, adopted as religion through a misunderstanding. Many scholars also used the word in its language sense for anything new and divided it into good and bad, while the blameworthy bid'ah in religion is always rejected.
The four schools agree on this.
Explanation
In his sermons the Prophet ﷺ would say: "The best speech is the Book of Allah, the best guidance is the guidance of Muhammad, the worst of matters are the newly invented ones, and every bid'ah is misguidance." He also said: "Whoever introduces into this affair of ours something that is not from it, it is rejected." The religion was completed in his lifetime: "This day I have perfected for you your religion" (5:3).
Scholars took two ways of speaking about it. Some, like Imam al-Shafi'i, al-Izz ibn Abd al-Salam and al-Nawawi, used bid'ah for anything new and sorted new things into the five rulings: some obligatory, such as writing grammar books to protect the Quran, some recommended, some disliked, some haram. They cited 'Umar's words about gathering people for taraweeh behind one imam: "What a good innovation this is." Others used bid'ah only for the blameworthy kind, and said good new things are not bid'ah at all because they have a basis in the Shariah. The difference is largely one of terms.
The Deobandi scholars stress a point that matters in daily life: making a permitted or recommended act seem obligatory, or tying it to fixed dates, forms and customs that the Sunnah did not set, and then blaming those who leave it, can turn it into a bid'ah. Respected scholars differ over some practices, such as celebrating the Mawlid; see Mawlid: scholarly views. Such differences should be discussed with knowledge and good manners, not insults.
The safe course is to hold to what the Prophet ﷺ and his Companions did, and to ask a scholar about anything new in worship. See what is the Sunnah? and what is aqeedah?
The evidence
Quran, Surah Al-Maaida 5:3
حُرِّمَتْ عَلَيْكُمُ ٱلْمَيْتَةُ وَٱلدَّمُ وَلَحْمُ ٱلْخِنزِيرِ وَمَآ أُهِلَّ لِغَيْرِ ٱللَّهِ بِهِۦ وَٱلْمُنْخَنِقَةُ وَٱلْمَوْقُوذَةُ وَٱلْمُتَرَدِّيَةُ وَٱلنَّطِيحَةُ وَمَآ أَكَلَ ٱلسَّبُعُ إِلَّا مَا ذَكَّيْتُمْ وَمَا ذُبِحَ عَلَى ٱلنُّصُبِ وَأَن تَسْتَقْسِمُوا۟ بِٱلْأَزْلَٰمِ ۚ ذَٰلِكُمْ فِسْقٌ ۗ ٱلْيَوْمَ يَئِسَ ٱلَّذِينَ كَفَرُوا۟ مِن دِينِكُمْ فَلَا تَخْشَوْهُمْ وَٱخْشَوْنِ ۚ ٱلْيَوْمَ أَكْمَلْتُ لَكُمْ دِينَكُمْ وَأَتْمَمْتُ عَلَيْكُمْ نِعْمَتِى وَرَضِيتُ لَكُمُ ٱلْإِسْلَٰمَ دِينًۭا ۚ فَمَنِ ٱضْطُرَّ فِى مَخْمَصَةٍ غَيْرَ مُتَجَانِفٍۢ لِّإِثْمٍۢ ۙ فَإِنَّ ٱللَّهَ غَفُورٌۭ رَّحِيمٌۭ ﴿٣﴾
Prohibited to you are dead animals, blood, the flesh of swine, and that which has been dedicated to other than Allah, and [those animals] killed by strangling or by a violent blow or by a head-long fall or by the goring of horns, and those from which a wild animal has eaten, except what you [are able to] slaughter [before its death], and those which are sacrificed on stone altars, and [prohibited is] that you seek decision through divining arrows. That is grave disobedience. This day those who disbelieve have despaired of [defeating] your religion; so fear them not, but fear Me. This day I have perfected for you your religion and completed My favor upon you and have approved for you Islam as religion. But whoever is forced by severe hunger with no inclination to sin - then indeed, Allah is Forgiving and Merciful.
Shows the religion was complete in the Prophet's lifetime.
Sahih Muslim 867
Sahih... وَشَرُّ الأُمُورِ مُحْدَثَاتُهَا وَكُلُّ بِدْعَةٍ ضَلاَلَةٌ ...
"The worst of matters are the newly invented ones, and every innovation is misguidance."
The main warning against innovation in religion.
Sahih al-Bukhari 2697
Sahih... مَنْ أَحْدَثَ فِي أَمْرِنَا هَذَا مَا لَيْسَ فِيهِ فَهُوَ رَدٌّ ...
"Whoever introduces into this affair of ours something that is not from it, it is rejected."
Shows that innovated acts of worship are not accepted.
Where the ruling is recorded
- al-Durr al-Mukhtar with Radd al-Muhtar, Book of prayer, chapter on leading the prayer. Defines bid'ah and discusses praying behind an innovator.
- Islah al-Rusum (Mawlana Ashraf Ali Thanawi), On customs added to religious occasions. Explains how customs become innovations when treated as part of the religion.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.